diff --git a/bench/download.cgi b/bench/download.cgi
index 359b8b0c..6726fcc0 100755
--- a/bench/download.cgi
+++ b/bench/download.cgi
@@ -15,7 +15,7 @@ end
 buf_size = 1024 * 16
 STDOUT.sync = true
 File.open(File.expand_path('10M.bin', File.dirname(__FILE__))) do |file|
-  buf = ''
+  buf = ''.dup
   while !file.read(buf_size, buf).nil?
     print dump_chunk(buf)
   end
diff --git a/lib/hexdump.rb b/lib/hexdump.rb
index 4cd006f3..aa3ef797 100644
--- a/lib/hexdump.rb
+++ b/lib/hexdump.rb
@@ -1,4 +1,5 @@
 # encoding: binary
+# frozen_string_literal: true
 
 # This was written by Arai-san and published at
 # http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-list/31987
@@ -11,7 +12,7 @@ def encode(str)
     result = []
     while raw = str.slice(offset, 16) and raw.length > 0
       # data field
-      data = ''
+      data = ''.dup
       for v in raw.unpack('N* a*')
 	if v.kind_of? Integer
 	  data << sprintf("%08x ", v)
diff --git a/lib/http-access2.rb b/lib/http-access2.rb
index 4588011d..81ab797f 100644
--- a/lib/http-access2.rb
+++ b/lib/http-access2.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # HTTPAccess2 - HTTP accessing library.
 # Copyright (C) 2000-2007  NAKAMURA, Hiroshi  <nakahiro@sarion.co.jp>.
 
diff --git a/lib/http-access2/cookie.rb b/lib/http-access2/cookie.rb
index 56f78842..73f6842e 100644
--- a/lib/http-access2/cookie.rb
+++ b/lib/http-access2/cookie.rb
@@ -1 +1,2 @@
+# frozen_string_literal: true
 require 'httpclient/cookie'
diff --git a/lib/http-access2/http.rb b/lib/http-access2/http.rb
index fc9b23c8..a564683f 100644
--- a/lib/http-access2/http.rb
+++ b/lib/http-access2/http.rb
@@ -1 +1,2 @@
+# frozen_string_literal: true
 require 'httpclient/http'
diff --git a/lib/httpclient.rb b/lib/httpclient.rb
index e1f18647..a0dcefd0 100644
--- a/lib/httpclient.rb
+++ b/lib/httpclient.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # HTTPClient - HTTP client library.
 # Copyright (C) 2000-2015  NAKAMURA, Hiroshi  <nahi@ruby-lang.org>.
 #
@@ -949,7 +951,7 @@ def request_async(method, uri, query = nil, body = nil, header = {})
   def request_async2(method, uri, *args)
     query, body, header = keyword_argument(args, :query, :body, :header)
     if [:post, :put].include?(method)
-      body ||= ''
+      body ||= ''.dup
     end
     if method == :propfind
       header ||= PROPFIND_DEFAULT_EXTHEADER
@@ -1240,7 +1242,7 @@ def do_get_block(req, proxy, conn, &block)
       conn.push(res)
       return res
     end
-    content = block ? nil : ''
+    content = block ? nil : ''.dup
     res = HTTP::Message.new_response(content, req.header)
     @debug_dev << "= Request\n\n" if @debug_dev
     sess = @session_manager.query(req, proxy)
diff --git a/lib/httpclient/auth.rb b/lib/httpclient/auth.rb
index 6b7b8b37..9eb1eca0 100644
--- a/lib/httpclient/auth.rb
+++ b/lib/httpclient/auth.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # HTTPClient - HTTP client library.
 # Copyright (C) 2000-2015  NAKAMURA, Hiroshi  <nahi@ruby-lang.org>.
 #
diff --git a/lib/httpclient/connection.rb b/lib/httpclient/connection.rb
index b702c928..9044c1cd 100644
--- a/lib/httpclient/connection.rb
+++ b/lib/httpclient/connection.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # HTTPClient - HTTP client library.
 # Copyright (C) 2000-2015  NAKAMURA, Hiroshi  <nahi@ruby-lang.org>.
 #
diff --git a/lib/httpclient/cookie.rb b/lib/httpclient/cookie.rb
index 65fc6caf..c4c76ea4 100644
--- a/lib/httpclient/cookie.rb
+++ b/lib/httpclient/cookie.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # do not override if httpclient/webagent-cookie is loaded already
 unless defined?(HTTPClient::CookieManager)
 begin # for catching LoadError and load webagent-cookie instead
diff --git a/lib/httpclient/http.rb b/lib/httpclient/http.rb
index f6a86079..4642acdb 100644
--- a/lib/httpclient/http.rb
+++ b/lib/httpclient/http.rb
@@ -1,4 +1,5 @@
 # -*- encoding: utf-8 -*-
+# frozen_string_literal: true
 
 # HTTPClient - HTTP client library.
 # Copyright (C) 2000-2015  NAKAMURA, Hiroshi  <nahi@ruby-lang.org>.
@@ -238,7 +239,7 @@ def content_type=(content_type)
       if defined?(Encoding::ASCII_8BIT)
         def set_body_encoding
           if type = self.content_type
-            OpenURI::Meta.init(o = '')
+            OpenURI::Meta.init(o = ''.dup)
             o.meta_add_field('content-type', type)
             @body_encoding = o.encoding
           end
@@ -491,7 +492,7 @@ def init_response(body = nil)
       # String.
       #
       # assert: @size is not nil
-      def dump(header = '', dev = '')
+      def dump(header = ''.dup, dev = ''.dup)
         if @body.is_a?(Parts)
           dev << header
           @body.parts.each do |part|
@@ -521,7 +522,7 @@ def dump(header = '', dev = '')
       # reason. (header is dumped to dev, too)
       # If no dev (the second argument) given, this method returns a dumped
       # String.
-      def dump_chunked(header = '', dev = '')
+      def dump_chunked(header = ''.dup, dev = ''.dup)
         dev << header
         if @body.is_a?(Parts)
           @body.parts.each do |part|
@@ -574,7 +575,7 @@ def reset_pos(io)
       end
 
       def dump_file(io, dev, sz)
-        buf = ''
+        buf = ''.dup
         rest = sz
         while rest > 0
           n = io.read([rest, @chunk_size].min, buf)
@@ -585,7 +586,7 @@ def dump_file(io, dev, sz)
       end
 
       def dump_chunks(io, dev)
-        buf = ''
+        buf = ''.dup
         while !io.read(@chunk_size, buf).nil?
           dev << dump_chunk(buf)
         end
@@ -954,7 +955,7 @@ def initialize # :nodoc:
 
     # Dumps message (header and body) to given dev.
     # dev needs to respond to <<.
-    def dump(dev = '')
+    def dump(dev = ''.dup)
       str = @http_header.dump + CRLF
       if @http_header.chunked
         dev = @http_body.dump_chunked(str, dev)
diff --git a/lib/httpclient/include_client.rb b/lib/httpclient/include_client.rb
index e6aaf47b..5b66905f 100644
--- a/lib/httpclient/include_client.rb
+++ b/lib/httpclient/include_client.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # It is useful to re-use a HTTPClient instance for multiple requests, to
 # re-use HTTP 1.1 persistent connections. 
 #
diff --git a/lib/httpclient/jruby_ssl_socket.rb b/lib/httpclient/jruby_ssl_socket.rb
index 1a0db8ea..9ce9f6eb 100644
--- a/lib/httpclient/jruby_ssl_socket.rb
+++ b/lib/httpclient/jruby_ssl_socket.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # HTTPClient - HTTP client library.
 # Copyright (C) 2000-2015  NAKAMURA, Hiroshi  <nahi@ruby-lang.org>.
 #
@@ -42,7 +44,7 @@ def initialize(socket, debug_dev = nil)
       @outstr = @socket.getOutputStream
       @instr = BufferedInputStream.new(@socket.getInputStream)
       @buf = (' ' * BUF_SIZE).to_java_bytes
-      @bufstr = ''
+      @bufstr = ''.dup
     end
 
     def close
@@ -346,7 +348,7 @@ def add(cert_source)
           File.read(cert_source).each_line do |line|
             case line
             when /-----BEGIN CERTIFICATE-----/
-              pem = ''
+              pem = ''.dup
             when /-----END CERTIFICATE-----/
               load_pem(pem)
               # keep parsing in case where multiple certificates in a file
diff --git a/lib/httpclient/session.rb b/lib/httpclient/session.rb
index 67e2c3ba..82e915d4 100644
--- a/lib/httpclient/session.rb
+++ b/lib/httpclient/session.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # HTTPClient - HTTP client library.
 # Copyright (C) 2000-2015  NAKAMURA, Hiroshi  <nahi@ruby-lang.org>.
 #
@@ -950,7 +952,7 @@ def read_body_rest
     end
 
     def empty_bin_str
-      str = ''
+      str = ''.dup
       str.force_encoding('BINARY') if str.respond_to?(:force_encoding)
       str
     end
diff --git a/lib/httpclient/ssl_config.rb b/lib/httpclient/ssl_config.rb
index 472db9d0..bd190928 100644
--- a/lib/httpclient/ssl_config.rb
+++ b/lib/httpclient/ssl_config.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # HTTPClient - HTTP client library.
 # Copyright (C) 2000-2015  NAKAMURA, Hiroshi  <nahi@ruby-lang.org>.
 #
diff --git a/lib/httpclient/ssl_socket.rb b/lib/httpclient/ssl_socket.rb
index 14801428..d05f8800 100644
--- a/lib/httpclient/ssl_socket.rb
+++ b/lib/httpclient/ssl_socket.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # HTTPClient - HTTP client library.
 # Copyright (C) 2000-2015  NAKAMURA, Hiroshi  <nahi@ruby-lang.org>.
 #
diff --git a/lib/httpclient/timeout.rb b/lib/httpclient/timeout.rb
index c5bbcacb..1f431104 100644
--- a/lib/httpclient/timeout.rb
+++ b/lib/httpclient/timeout.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # HTTPClient - HTTP client library.
 # Copyright (C) 2000-2015  NAKAMURA, Hiroshi  <nahi@ruby-lang.org>.
 #
diff --git a/lib/httpclient/util.rb b/lib/httpclient/util.rb
index 6ff38016..58eb8c11 100644
--- a/lib/httpclient/util.rb
+++ b/lib/httpclient/util.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # HTTPClient - HTTP client library.
 # Copyright (C) 2000-2015  NAKAMURA, Hiroshi  <nahi@ruby-lang.org>.
 #
@@ -64,7 +66,7 @@ class AddressableURI < Addressable::URI
         # Overwrites the original definition just for one line...
         def authority
           self.host && @authority ||= (begin
-            authority = ""
+            authority = "".dup
             if self.userinfo != nil
               authority << "#{self.userinfo}@"
             end
diff --git a/lib/httpclient/version.rb b/lib/httpclient/version.rb
index d1ab8e67..9737d671 100644
--- a/lib/httpclient/version.rb
+++ b/lib/httpclient/version.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class HTTPClient
   VERSION = '2.8.3'
 end
diff --git a/lib/httpclient/webagent-cookie.rb b/lib/httpclient/webagent-cookie.rb
index 74dbfef9..d5859239 100644
--- a/lib/httpclient/webagent-cookie.rb
+++ b/lib/httpclient/webagent-cookie.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # cookie.rb is redistributed file which is originally included in Webagent
 # version 0.6.2 by TAKAHASHI `Maki' Masayoshi.  And it contains some bug fixes.
 # You can download the entire package of Webagent from
diff --git a/lib/jsonclient.rb b/lib/jsonclient.rb
index cf1ef7dc..f322539f 100644
--- a/lib/jsonclient.rb
+++ b/lib/jsonclient.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'httpclient'
 require 'json'
  
diff --git a/lib/oauthclient.rb b/lib/oauthclient.rb
index f9bb9d06..4364cb47 100644
--- a/lib/oauthclient.rb
+++ b/lib/oauthclient.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 # HTTPClient - HTTP client library.
 # Copyright (C) 2000-2015  NAKAMURA, Hiroshi  <nahi@ruby-lang.org>.
 #
diff --git a/test/helper.rb b/test/helper.rb
index 26bc4f9b..5d544514 100644
--- a/test/helper.rb
+++ b/test/helper.rb
@@ -1,4 +1,6 @@
 # -*- encoding: utf-8 -*-
+# frozen_string_literal: true
+
 begin
   require 'simplecov'
   require 'simplecov-rcov'
diff --git a/test/jruby_ssl_socket/test_pemutils.rb b/test/jruby_ssl_socket/test_pemutils.rb
index 560a1c6e..33b9a73d 100644
--- a/test/jruby_ssl_socket/test_pemutils.rb
+++ b/test/jruby_ssl_socket/test_pemutils.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require File.expand_path('helper', File.join(File.dirname(__FILE__),  ".."))
 
 
diff --git a/test/runner.rb b/test/runner.rb
index a07c28e6..07ea2e88 100644
--- a/test/runner.rb
+++ b/test/runner.rb
@@ -1,2 +1,3 @@
+# frozen_string_literal: true
 require 'test/unit'
 exit Test::Unit::AutoRunner.run(true, File.dirname($0))
diff --git a/test/sslsvr.rb b/test/sslsvr.rb
index b1f46143..c968ae3a 100644
--- a/test/sslsvr.rb
+++ b/test/sslsvr.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require 'webrick/https'
 require 'logger'
 require 'rbconfig'
diff --git a/test/test_auth.rb b/test/test_auth.rb
index 1c1367fb..8a497258 100644
--- a/test/test_auth.rb
+++ b/test/test_auth.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require File.expand_path('helper', File.dirname(__FILE__))
 require 'digest/md5'
 require 'rack'
@@ -176,7 +177,7 @@ def test_BASIC_auth_force
       c.www_auth.basic_auth.instance_eval { @scheme = "BASIC" }
       #
       c.force_basic_auth = true
-      c.debug_dev = str = ''
+      c.debug_dev = str = ''.dup
       c.set_auth("http://localhost:#{serverport}/", 'admin', 'admin')
       assert_equal('basic_auth OK', c.get_content("http://localhost:#{serverport}/basic_auth"))
       assert_equal('Authorization: Basic YWRtaW46YWRtaW4='.upcase, str.split(/\r?\n/)[5].upcase)
@@ -249,7 +250,7 @@ def test_basic_auth_reuses_credentials
     c.set_auth("http://localhost:#{serverport}/", 'admin', 'admin')
     assert_equal('basic_auth OK', c.get_content("http://localhost:#{serverport}/basic_auth/"))
     c.test_loopback_http_response << "HTTP/1.0 200 OK\nContent-Length: 2\n\nOK"
-    c.debug_dev = str = ''
+    c.debug_dev = str = ''.dup
     c.get_content("http://localhost:#{serverport}/basic_auth/sub/dir/")
     assert_match(/Authorization: Basic YWRtaW46YWRtaW4=/, str)
   end
@@ -265,7 +266,7 @@ def test_digest_auth_reuses_credentials
     c.set_auth("http://localhost:#{serverport}/", 'admin', 'admin')
     assert_equal('digest_auth OK', c.get_content("http://localhost:#{serverport}/digest_auth/"))
     c.test_loopback_http_response << "HTTP/1.0 200 OK\nContent-Length: 2\n\nOK"
-    c.debug_dev = str = ''
+    c.debug_dev = str = ''.dup
     c.get_content("http://localhost:#{serverport}/digest_auth/sub/dir/")
     assert_match(/Authorization: Digest/, str)
   end
@@ -311,7 +312,7 @@ def test_perfer_digest
     c.set_auth('http://example.com/', 'admin', 'admin')
     c.test_loopback_http_response << "HTTP/1.0 401 Unauthorized\nWWW-Authenticate: Basic realm=\"foo\"\nWWW-Authenticate: Digest realm=\"foo\", nonce=\"nonce\", stale=false\nContent-Length: 2\n\nNG"
     c.test_loopback_http_response << "HTTP/1.0 200 OK\nContent-Length: 2\n\nOK"
-    c.debug_dev = str = ''
+    c.debug_dev = str = ''.dup
     c.get_content('http://example.com/')
     assert_match(/^Authorization: Digest/, str)
   end
@@ -327,7 +328,7 @@ def test_proxy_auth
     c.set_proxy_auth('admin', 'admin')
     c.test_loopback_http_response << "HTTP/1.0 407 Unauthorized\nProxy-Authenticate: Basic realm=\"foo\"\nContent-Length: 2\n\nNG"
     c.test_loopback_http_response << "HTTP/1.0 200 OK\nContent-Length: 2\n\nOK"
-    c.debug_dev = str = ''
+    c.debug_dev = str = ''.dup
     c.get_content('http://example.com/')
     assert_match(/Proxy-Authorization: Basic YWRtaW46YWRtaW4=/, str)
   end
@@ -337,7 +338,7 @@ def test_proxy_auth_force
     c.set_proxy_auth('admin', 'admin')
     c.force_basic_auth = true
     c.test_loopback_http_response << "HTTP/1.0 200 OK\nContent-Length: 2\n\nOK"
-    c.debug_dev = str = ''
+    c.debug_dev = str = ''.dup
     c.get_content('http://example.com/')
     assert_match(/Proxy-Authorization: Basic YWRtaW46YWRtaW4=/, str)
   end
@@ -349,7 +350,7 @@ def test_proxy_auth_reuses_credentials
     c.test_loopback_http_response << "HTTP/1.0 200 OK\nContent-Length: 2\n\nOK"
     c.test_loopback_http_response << "HTTP/1.0 200 OK\nContent-Length: 2\n\nOK"
     c.get_content('http://www1.example.com/')
-    c.debug_dev = str = ''
+    c.debug_dev = str = ''.dup
     c.get_content('http://www2.example.com/')
     assert_match(/Proxy-Authorization: Basic YWRtaW46YWRtaW4=/, str)
   end
@@ -363,7 +364,7 @@ def test_digest_proxy_auth_loop
     ha1 = md5.hexdigest("admin:foo:admin")
     ha2 = md5.hexdigest("GET:/")
     response = md5.hexdigest("#{ha1}:nonce:#{ha2}")
-    c.debug_dev = str = ''
+    c.debug_dev = str = ''.dup
     c.get_content('http://example.com/')
     assert_match(/Proxy-Authorization: Digest/, str)
     assert_match(%r"response=\"#{response}\"", str)
@@ -394,7 +395,7 @@ def test_prefer_digest_to_basic_proxy_auth
     ha1 = md5.hexdigest("admin:foo:admin")
     ha2 = md5.hexdigest("GET:/")
     response = md5.hexdigest("#{ha1}:nonce:#{ha2}")
-    c.debug_dev = str = ''
+    c.debug_dev = str = ''.dup
     c.get_content('http://example.com/')
     assert_match(/Proxy-Authorization: Digest/, str)
     assert_match(%r"response=\"#{response}\"", str)
@@ -411,7 +412,7 @@ def test_digest_proxy_auth_reuses_credentials
     ha2 = md5.hexdigest("GET:/")
     response = md5.hexdigest("#{ha1}:nonce:#{ha2}")
     c.get_content('http://www1.example.com/')
-    c.debug_dev = str = ''
+    c.debug_dev = str = ''.dup
     c.get_content('http://www2.example.com/')
     assert_match(/Proxy-Authorization: Digest/, str)
     assert_match(%r"response=\"#{response}\"", str)
@@ -433,19 +434,19 @@ def test_oauth
     c.www_auth.oauth.set_config('http://photos.example.net/', config)
     c.www_auth.oauth.challenge('http://photos.example.net/')
     c.test_loopback_http_response << "HTTP/1.0 200 OK\nContent-Length: 2\n\nOK"
-    c.debug_dev = str = ''
+    c.debug_dev = str = ''.dup
     c.get_content('http://photos.example.net/photos', [[:file, 'vacation.jpg'], [:size, 'original']])
     assert(str.index(%q(GET /photos?file=vacation.jpg&size=original)))
     assert(str.index(%q(Authorization: OAuth realm="http://photos.example.net/", oauth_consumer_key="dpf43f3p2l4k3l03", oauth_nonce="kllo9940pd9333jh", oauth_signature="tR3%2BTy81lMeYAr%2FFid0kMTYa%2FWM%3D", oauth_signature_method="HMAC-SHA1", oauth_timestamp="1191242096", oauth_token="nnch734d00sl2jdk", oauth_version="1.0")))
     #
     c.test_loopback_http_response << "HTTP/1.0 200 OK\nContent-Length: 2\n\nOK"
-    c.debug_dev = str = ''
+    c.debug_dev = str = ''.dup
     c.get_content('http://photos.example.net/photos?file=vacation.jpg&size=original')
     assert(str.index(%q(GET /photos?file=vacation.jpg&size=original)))
     assert(str.index(%q(Authorization: OAuth realm="http://photos.example.net/", oauth_consumer_key="dpf43f3p2l4k3l03", oauth_nonce="kllo9940pd9333jh", oauth_signature="tR3%2BTy81lMeYAr%2FFid0kMTYa%2FWM%3D", oauth_signature_method="HMAC-SHA1", oauth_timestamp="1191242096", oauth_token="nnch734d00sl2jdk", oauth_version="1.0")))
     #
     c.test_loopback_http_response << "HTTP/1.0 200 OK\nContent-Length: 2\n\nOK"
-    c.debug_dev = str = ''
+    c.debug_dev = str = ''.dup
     c.post_content('http://photos.example.net/photos', [[:file, 'vacation.jpg'], [:size, 'original']])
     assert(str.index(%q(POST /photos)))
     assert(str.index(%q(Authorization: OAuth realm="http://photos.example.net/", oauth_consumer_key="dpf43f3p2l4k3l03", oauth_nonce="kllo9940pd9333jh", oauth_signature="wPkvxykrw%2BBTdCcGqKr%2B3I%2BPsiM%3D", oauth_signature_method="HMAC-SHA1", oauth_timestamp="1191242096", oauth_token="nnch734d00sl2jdk", oauth_version="1.0")))
@@ -475,7 +476,7 @@ def test_negotiate_and_basic
     c.test_loopback_http_response << %Q(HTTP/1.1 401 Unauthorized\r\nWWW-Authenticate: NTLM TlRMTVNTUAACAAAAAAAAACgAAAABAAAAAAAAAAAAAAA=\r\nConnection: Keep-Alive\r\nContent-Length: 0\r\n\r\n)
     c.test_loopback_http_response << %Q(HTTP/1.0 200 OK\r\nConnection: Keep-Alive\r\nContent-Length: 1\r\n\r\na)
     c.test_loopback_http_response << %Q(HTTP/1.0 200 OK\r\nConnection: Keep-Alive\r\nContent-Length: 1\r\n\r\nb)
-    c.debug_dev = str = ''
+    c.debug_dev = str = ''.dup
     c.set_auth('http://www.example.org/', 'admin', 'admin')
     # Do NTLM negotiation
     c.get('http://www.example.org/foo')
@@ -484,7 +485,7 @@ def test_negotiate_and_basic
     assert_match(%r(Authorization: NTLM), str)
     assert_not_match(%r(Authorization: Basic), str)
     # ditto for other resource that is protected with NTLM
-    c.debug_dev = str = ''
+    c.debug_dev = str = ''.dup
     c.get('http://www.example.org/foo/subdir')
     assert_not_match(%r(Authorization: NTLM), str)
     assert_not_match(%r(Authorization: Basic), str)
diff --git a/test/test_cookie.rb b/test/test_cookie.rb
index 57be1e07..ecf7a0e3 100644
--- a/test/test_cookie.rb
+++ b/test/test_cookie.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require 'test/unit'
 require 'uri'
 require 'tempfile'
diff --git a/test/test_hexdump.rb b/test/test_hexdump.rb
index 7e4c5649..922c29c5 100644
--- a/test/test_hexdump.rb
+++ b/test/test_hexdump.rb
@@ -1,11 +1,12 @@
 # -*- encoding: utf-8 -*-
+# frozen_string_literal: true
 require File.expand_path('helper', File.dirname(__FILE__))
 require 'hexdump'
 
 
 class TestHexDump < Test::Unit::TestCase
   def test_encode
-    str = "\032l\277\370\2429\216\236\351[{\{\262\350\274\376"
+    str = "\032l\277\370\2429\216\236\351[{\{\262\350\274\376".dup
     str.force_encoding('BINARY') if str.respond_to?(:force_encoding)
     assert_equal(["00000000  1a6cbff8 a2398e9e e95b7b7b b2e8bcfe   .l...9...[{{...."], HexDump.encode(str))
   end
diff --git a/test/test_http-access2.rb b/test/test_http-access2.rb
index 732c0443..11b7c6d9 100644
--- a/test/test_http-access2.rb
+++ b/test/test_http-access2.rb
@@ -1,4 +1,5 @@
 # -*- encoding: utf-8 -*-
+# frozen_string_literal: true
 require 'http-access2'
 require File.expand_path('helper', File.dirname(__FILE__))
 require 'tempfile'
@@ -24,7 +25,7 @@ def teardown
   def test_initialize
     setup_proxyserver
     escape_noproxy do
-      @proxyio.string = ""
+      @proxyio.string = "".dup
       @client = HTTPAccess2::Client.new(proxyurl)
       assert_equal(urify(proxyurl), @client.proxy)
       assert_equal(200, @client.head(serverurl).status)
@@ -34,7 +35,7 @@ def test_initialize
 
   def test_agent_name
     @client = HTTPAccess2::Client.new(nil, "agent_name_foo")
-    str = ""
+    str = "".dup
     @client.debug_dev = str
     @client.get(serverurl)
     lines = str.split(/(?:\r?\n)+/)
@@ -44,7 +45,7 @@ def test_agent_name
 
   def test_from
     @client = HTTPAccess2::Client.new(nil, nil, "from_bar")
-    str = ""
+    str = "".dup
     @client.debug_dev = str
     @client.get(serverurl)
     lines = str.split(/(?:\r?\n)+/)
@@ -53,7 +54,7 @@ def test_from
   end
 
   def test_debug_dev
-    str = ""
+    str = "".dup
     @client.debug_dev = str
     assert(str.empty?)
     @client.get(serverurl)
@@ -62,7 +63,7 @@ def test_debug_dev
 
   def _test_protocol_version_http09
     @client.protocol_version = 'HTTP/0.9'
-    str = ""
+    str = "".dup
     @client.debug_dev = str
     @client.get(serverurl + 'hello')
     lines = str.split(/(?:\r?\n)+/)
@@ -76,7 +77,7 @@ def _test_protocol_version_http09
 
   def test_protocol_version_http10
     @client.protocol_version = 'HTTP/1.0'
-    str = ""
+    str = "".dup
     @client.debug_dev = str
     @client.get(serverurl + 'hello')
     lines = str.split(/(?:\r?\n)+/)
@@ -88,7 +89,7 @@ def test_protocol_version_http10
   end
 
   def test_protocol_version_http11
-    str = ""
+    str = "".dup
     @client.debug_dev = str
     @client.get(serverurl)
     lines = str.split(/(?:\r?\n)+/)
@@ -97,7 +98,7 @@ def test_protocol_version_http11
     assert_equal("GET / HTTP/1.1", lines[3])
     assert_equal("Host: localhost:#{serverport}", lines[7])
     @client.protocol_version = 'HTTP/1.1'
-    str = ""
+    str = "".dup
     @client.debug_dev = str
     @client.get(serverurl)
     lines = str.split(/(?:\r?\n)+/)
@@ -105,7 +106,7 @@ def test_protocol_version_http11
     assert_equal("! CONNECTION ESTABLISHED", lines[2])
     assert_equal("GET / HTTP/1.1", lines[3])
     @client.protocol_version = 'HTTP/1.0'
-    str = ""
+    str = "".dup
     @client.debug_dev = str
     @client.get(serverurl)
     lines = str.split(/(?:\r?\n)+/)
@@ -122,7 +123,7 @@ def test_proxy
       rescue => e
         assert_match(/InvalidURIError/, e.class.to_s)
       end
-      @client.proxy = ""
+      @client.proxy = "".dup
       assert_nil(@client.proxy)
       @client.proxy = "http://foo:1234"
       assert_equal(urify("http://foo:1234"), @client.proxy)
@@ -130,12 +131,12 @@ def test_proxy
       @client.proxy = uri
       assert_equal(uri, @client.proxy)
       #
-      @proxyio.string = ""
+      @proxyio.string = "".dup
       @client.proxy = nil
       assert_equal(200, @client.head(serverurl).status)
       assert(@proxyio.string.empty?)
       #
-      @proxyio.string = ""
+      @proxyio.string = "".dup
       @client.proxy = proxyurl
       assert_equal(200, @client.head(serverurl).status)
       assert(!@proxyio.string.empty?)
@@ -143,7 +144,7 @@ def test_proxy
   end
 
   def test_noproxy_for_localhost
-    @proxyio.string = ""
+    @proxyio.string = "".dup
     @client.proxy = proxyurl
     assert_equal(200, @client.head(serverurl).status)
     assert(@proxyio.string.empty?)
@@ -154,36 +155,36 @@ def test_no_proxy
     escape_noproxy do
       # proxy is not set.
       @client.no_proxy = 'localhost'
-      @proxyio.string = ""
+      @proxyio.string = "".dup
       @client.proxy = nil
       assert_equal(200, @client.head(serverurl).status)
       assert(/accept/ !~ @proxyio.string)
       #
-      @proxyio.string = ""
+      @proxyio.string = "".dup
       @client.proxy = proxyurl
       assert_equal(200, @client.head(serverurl).status)
       assert(/accept/ !~ @proxyio.string)
       #
       @client.no_proxy = 'foobar'
-      @proxyio.string = ""
+      @proxyio.string = "".dup
       @client.proxy = proxyurl
       assert_equal(200, @client.head(serverurl).status)
       assert(/accept/ =~ @proxyio.string)
       #
       @client.no_proxy = 'foobar,localhost:baz'
-      @proxyio.string = ""
+      @proxyio.string = "".dup
       @client.proxy = proxyurl
       assert_equal(200, @client.head(serverurl).status)
       assert(/accept/ !~ @proxyio.string)
       #
       @client.no_proxy = 'foobar,localhost:443'
-      @proxyio.string = ""
+      @proxyio.string = "".dup
       @client.proxy = proxyurl
       assert_equal(200, @client.head(serverurl).status)
       assert(/accept/ =~ @proxyio.string)
       #
       @client.no_proxy = "foobar,localhost:443:localhost:#{serverport},baz"
-      @proxyio.string = ""
+      @proxyio.string = "".dup
       @client.proxy = proxyurl
       assert_equal(200, @client.head(serverurl).status)
       assert(/accept/ !~ @proxyio.string)
@@ -314,14 +315,14 @@ def test_post_body
   end
 
   def test_extra_headers
-    str = ""
+    str = "".dup
     @client.debug_dev = str
     @client.head(serverurl, nil, {"ABC" => "DEF"})
     lines = str.split(/(?:\r?\n)+/)
     assert_equal("= Request", lines[0])
     assert_match("ABC: DEF", lines[4])
     #
-    str = ""
+    str = "".dup
     @client.debug_dev = str
     @client.get(serverurl, nil, [["ABC", "DEF"], ["ABC", "DEF"]])
     lines = str.split(/(?:\r?\n)+/)
diff --git a/test/test_httpclient.rb b/test/test_httpclient.rb
index c8e5330c..cb5544a9 100644
--- a/test/test_httpclient.rb
+++ b/test/test_httpclient.rb
@@ -1,4 +1,5 @@
 # -*- encoding: utf-8 -*-
+# frozen_string_literal: true
 require File.expand_path('helper', File.dirname(__FILE__))
 require 'tempfile'
 
@@ -20,7 +21,7 @@ def teardown
   def test_initialize
     setup_proxyserver
     escape_noproxy do
-      @proxyio.string = ""
+      @proxyio.string = "".dup
       @client = HTTPClient.new(proxyurl)
       assert_equal(urify(proxyurl), @client.proxy)
       assert_equal(200, @client.head(serverurl).status)
@@ -30,7 +31,7 @@ def test_initialize
 
   def test_agent_name
     @client = HTTPClient.new(nil, "agent_name_foo")
-    str = ""
+    str = "".dup
     @client.debug_dev = str
     @client.get(serverurl)
     lines = str.split(/(?:\r?\n)+/)
@@ -40,7 +41,7 @@ def test_agent_name
 
   def test_from
     @client = HTTPClient.new(nil, nil, "from_bar")
-    str = ""
+    str = "".dup
     @client.debug_dev = str
     @client.get(serverurl)
     lines = str.split(/(?:\r?\n)+/)
@@ -49,7 +50,7 @@ def test_from
   end
 
   def test_debug_dev
-    str = ""
+    str = "".dup
     @client.debug_dev = str
     assert_equal(str.object_id, @client.debug_dev.object_id)
     assert(str.empty?)
@@ -58,7 +59,7 @@ def test_debug_dev
   end
 
   def test_debug_dev_stream
-    str = ""
+    str = "".dup
     @client.debug_dev = str
     conn = @client.get_async(serverurl)
     Thread.pass while !conn.finished?
@@ -67,7 +68,7 @@ def test_debug_dev_stream
 
   def test_protocol_version_http09
     @client.protocol_version = 'HTTP/0.9'
-    @client.debug_dev = str = ''
+    @client.debug_dev = str = ''.dup
     @client.test_loopback_http_response << "hello\nworld\n"
     res = @client.get(serverurl + 'hello')
     assert_equal('0.9', res.http_version)
@@ -88,7 +89,7 @@ def test_protocol_version_http10
     assert_equal(nil, @client.protocol_version)
     @client.protocol_version = 'HTTP/1.0'
     assert_equal('HTTP/1.0', @client.protocol_version)
-    str = ""
+    str = "".dup
     @client.debug_dev = str
     @client.get(serverurl + 'hello')
     lines = str.split(/(?:\r?\n)+/)
@@ -100,7 +101,7 @@ def test_protocol_version_http10
   end
 
   def test_header_accept_by_default
-    str = ""
+    str = "".dup
     @client.debug_dev = str
     @client.get(serverurl)
     lines = str.split(/(?:\r?\n)+/)
@@ -108,7 +109,7 @@ def test_header_accept_by_default
   end
 
   def test_header_accept
-    str = ""
+    str = "".dup
     @client.debug_dev = str
     @client.get(serverurl, :header => {:Accept => 'text/html'})
     lines = str.split(/(?:\r?\n)+/)
@@ -116,7 +117,7 @@ def test_header_accept
   end
 
   def test_header_symbol
-    str = ""
+    str = "".dup
     @client.debug_dev = str
     @client.post(serverurl + 'servlet', :header => {:'Content-Type' => 'application/json'}, :body => 'hello')
     lines = str.split(/(?:\r?\n)+/).grep(/^Content-Type/)
@@ -124,7 +125,7 @@ def test_header_symbol
   end
 
   def test_host_given
-    str = ""
+    str = "".dup
     @client.debug_dev = str
     @client.get(serverurl)
     lines = str.split(/(?:\r?\n)+/)
@@ -134,7 +135,7 @@ def test_host_given
     assert_equal("Host: localhost:#{serverport}", lines[7])
     #
     @client.reset_all
-    str = ""
+    str = "".dup
     @client.debug_dev = str
     @client.get(serverurl, nil, {'Host' => 'foo'})
     lines = str.split(/(?:\r?\n)+/)
@@ -166,7 +167,7 @@ def test_redirect_without_location_should_gracefully_fail
 
   def test_protocol_version_http11
     assert_equal(nil, @client.protocol_version)
-    str = ""
+    str = "".dup
     @client.debug_dev = str
     @client.get(serverurl)
     lines = str.split(/(?:\r?\n)+/)
@@ -176,7 +177,7 @@ def test_protocol_version_http11
     assert_equal("Host: localhost:#{serverport}", lines[7])
     @client.protocol_version = 'HTTP/1.1'
     assert_equal('HTTP/1.1', @client.protocol_version)
-    str = ""
+    str = "".dup
     @client.debug_dev = str
     @client.get(serverurl)
     lines = str.split(/(?:\r?\n)+/)
@@ -184,7 +185,7 @@ def test_protocol_version_http11
     assert_equal("! CONNECTION ESTABLISHED", lines[2])
     assert_equal("GET / HTTP/1.1", lines[3])
     @client.protocol_version = 'HTTP/1.0'
-    str = ""
+    str = "".dup
     @client.debug_dev = str
     @client.get(serverurl)
     lines = str.split(/(?:\r?\n)+/)
@@ -201,7 +202,7 @@ def test_proxy
       rescue => e
         assert_match(/InvalidURIError/, e.class.to_s)
       end
-      @client.proxy = ""
+      @client.proxy = "".dup
       assert_nil(@client.proxy)
       @client.proxy = "http://admin:admin@foo:1234"
       assert_equal(urify("http://admin:admin@foo:1234"), @client.proxy)
@@ -209,14 +210,14 @@ def test_proxy
       @client.proxy = uri
       assert_equal(uri, @client.proxy)
       #
-      @proxyio.string = ""
+      @proxyio.string = "".dup
       @client.proxy = nil
       assert_equal(200, @client.head(serverurl).status)
       assert(/accept/ !~ @proxyio.string)
       #
-      @proxyio.string = ""
+      @proxyio.string = "".dup
       @client.proxy = proxyurl
-      @client.debug_dev = str = ""
+      @client.debug_dev = str = "".dup
       assert_equal(200, @client.head(serverurl).status)
       assert(/accept/ =~ @proxyio.string)
       assert(/Host: localhost:#{serverport}/ =~ str)
@@ -225,13 +226,13 @@ def test_proxy
 
   def test_host_header
     @client.proxy = proxyurl
-    @client.debug_dev = str = ""
+    @client.debug_dev = str = "".dup
     @client.test_loopback_http_response << "HTTP/1.0 200 OK\r\n\r\n"
     assert_equal(200, @client.head('http://www.example.com/foo').status)
     # ensure no ':80' is added.  some servers dislike that.
     assert(/\r\nHost: www\.example\.com\r\n/ =~ str)
     #
-    @client.debug_dev = str = ""
+    @client.debug_dev = str = "".dup
     @client.test_loopback_http_response << "HTTP/1.0 200 OK\r\n\r\n"
     assert_equal(200, @client.head('http://www.example.com:12345/foo').status)
     # ensure ':12345' exists.
@@ -266,14 +267,14 @@ def test_proxy_env_cgi
   def test_empty_proxy_env
     setup_proxyserver
     escape_env do
-      ENV['http_proxy'] = ""
+      ENV['http_proxy'] = "".dup
       client = HTTPClient.new
       assert_equal(nil, client.proxy)
     end
   end
 
   def test_noproxy_for_localhost
-    @proxyio.string = ""
+    @proxyio.string = "".dup
     @client.proxy = proxyurl
     assert_equal(200, @client.head(serverurl).status)
     assert(/accept/ !~ @proxyio.string)
@@ -286,36 +287,36 @@ def test_no_proxy
       assert_equal(nil, @client.no_proxy)
       @client.no_proxy = 'localhost'
       assert_equal('localhost', @client.no_proxy)
-      @proxyio.string = ""
+      @proxyio.string = "".dup
       @client.proxy = nil
       assert_equal(200, @client.head(serverurl).status)
       assert(/accept/ !~ @proxyio.string)
       #
-      @proxyio.string = ""
+      @proxyio.string = "".dup
       @client.proxy = proxyurl
       assert_equal(200, @client.head(serverurl).status)
       assert(/accept/ !~ @proxyio.string)
       #
       @client.no_proxy = 'foobar'
-      @proxyio.string = ""
+      @proxyio.string = "".dup
       @client.proxy = proxyurl
       assert_equal(200, @client.head(serverurl).status)
       assert(/accept/ =~ @proxyio.string)
       #
       @client.no_proxy = 'foobar,localhost:baz'
-      @proxyio.string = ""
+      @proxyio.string = "".dup
       @client.proxy = proxyurl
       assert_equal(200, @client.head(serverurl).status)
       assert(/accept/ !~ @proxyio.string)
       #
       @client.no_proxy = 'foobar,localhost:443'
-      @proxyio.string = ""
+      @proxyio.string = "".dup
       @client.proxy = proxyurl
       assert_equal(200, @client.head(serverurl).status)
       assert(/accept/ =~ @proxyio.string)
       #
       @client.no_proxy = "foobar,localhost:443:localhost:#{serverport},baz"
-      @proxyio.string = ""
+      @proxyio.string = "".dup
       @client.proxy = proxyurl
       assert_equal(200, @client.head(serverurl).status)
       assert(/accept/ !~ @proxyio.string)
@@ -323,28 +324,28 @@ def test_no_proxy
   end
 
   def test_no_proxy_with_initial_dot
-    @client.debug_dev = str = ""
+    @client.debug_dev = str = "".dup
     @client.test_loopback_http_response << "HTTP/1.0 200 OK\r\n\r\n"
     @client.no_proxy = ''
     @client.proxy = proxyurl
     @client.head('http://www.foo.com')
     assert(/CONNECT TO localhost/ =~ str, 'via proxy')
     #
-    @client.debug_dev = str = ""
+    @client.debug_dev = str = "".dup
     @client.test_loopback_http_response << "HTTP/1.0 200 OK\r\n\r\n"
     @client.no_proxy = '.foo.com'
     @client.proxy = proxyurl
     @client.head('http://www.foo.com')
     assert(/CONNECT TO www.foo.com/ =~ str, 'no proxy because .foo.com matches with www.foo.com')
     #
-    @client.debug_dev = str = ""
+    @client.debug_dev = str = "".dup
     @client.test_loopback_http_response << "HTTP/1.0 200 OK\r\n\r\n"
     @client.no_proxy = '.foo.com'
     @client.proxy = proxyurl
     @client.head('http://foo.com')
     assert(/CONNECT TO localhost/ =~ str, 'via proxy because .foo.com does not matche with foo.com')
     #
-    @client.debug_dev = str = ""
+    @client.debug_dev = str = "".dup
     @client.test_loopback_http_response << "HTTP/1.0 200 OK\r\n\r\n"
     @client.no_proxy = 'foo.com'
     @client.proxy = proxyurl
@@ -370,7 +371,7 @@ def test_cookie_update_while_authentication
 \r
 hello
 EOS
-      @client.debug_dev = str = ''
+      @client.debug_dev = str = ''.dup
       @client.set_auth("http://www.example.org/baz/", 'admin', 'admin')
       assert_equal('hello', @client.get('http://www.example.org/baz/foo').content)
       assert_match(/^Cookie: foo=bar/, str)
@@ -411,7 +412,7 @@ def test_loopback_response
     assert_equal('message body 1', @client.get_content('http://somewhere'))
     assert_equal('message body 2', @client.get_content('http://somewhere'))
     #
-    @client.debug_dev = str = ''
+    @client.debug_dev = str = ''.dup
     @client.test_loopback_response << 'message body 3'
     assert_equal('message body 3', @client.get_content('http://somewhere'))
     assert_match(/message body 3/, str)
@@ -597,9 +598,9 @@ def test_get_content_with_base_url
     assert(called)
   end
 
-  GZIP_CONTENT = "\x1f\x8b\x08\x00\x1a\x96\xe0\x4c\x00\x03\xcb\x48\xcd\xc9\xc9\x07\x00\x86\xa6\x10\x36\x05\x00\x00\x00"
-  DEFLATE_CONTENT = "\x78\x9c\xcb\x48\xcd\xc9\xc9\x07\x00\x06\x2c\x02\x15"
-  DEFLATE_NOHEADER_CONTENT = "x\x9C\xCBH\xCD\xC9\xC9\a\x00\x06,\x02\x15"
+  GZIP_CONTENT = "\x1f\x8b\x08\x00\x1a\x96\xe0\x4c\x00\x03\xcb\x48\xcd\xc9\xc9\x07\x00\x86\xa6\x10\x36\x05\x00\x00\x00".dup
+  DEFLATE_CONTENT = "\x78\x9c\xcb\x48\xcd\xc9\xc9\x07\x00\x06\x2c\x02\x15".dup
+  DEFLATE_NOHEADER_CONTENT = "x\x9C\xCBH\xCD\xC9\xC9\a\x00\x06,\x02\x15".dup
   [GZIP_CONTENT, DEFLATE_CONTENT, DEFLATE_NOHEADER_CONTENT].each do |content|
     content.force_encoding('BINARY') if content.respond_to?(:force_encoding)
   end
@@ -867,7 +868,7 @@ def test_post
   end
 
   def test_post_empty
-    @client.debug_dev = str = ''
+    @client.debug_dev = str = ''.dup
     # nil body means 'no content' that is allowed but WEBrick cannot handle it.
     @client.post(serverurl + 'servlet', :body => nil)
     # request does not have 'Content-Type'
@@ -1092,7 +1093,7 @@ def file.original_filename
   def test_patch
     assert_equal("patch", @client.patch(serverurl + 'servlet', '').content)
     param = {'1'=>'2', '3'=>'4'}
-    @client.debug_dev = str = ''
+    @client.debug_dev = str = ''.dup
     res = @client.patch(serverurl + 'servlet', param)
     assert_equal(param, params(res.header["x-query"][0]))
     assert_equal('Content-Type: application/x-www-form-urlencoded', str.split(/\r?\n/)[5])
@@ -1122,7 +1123,7 @@ def test_patch_async
   def test_put
     assert_equal("put", @client.put(serverurl + 'servlet', '').content)
     param = {'1'=>'2', '3'=>'4'}
-    @client.debug_dev = str = ''
+    @client.debug_dev = str = ''.dup
     res = @client.put(serverurl + 'servlet', param)
     assert_equal(param, params(res.header["x-query"][0]))
     assert_equal('Content-Type: application/x-www-form-urlencoded', str.split(/\r?\n/)[5])
@@ -1169,7 +1170,7 @@ def test_delete_with_query_and_body
   # Not prohibited by spec, but normally it's ignored
   def test_delete_with_body
     param = {'1'=>'2', '3'=>'4'}
-    @client.debug_dev = str = ''
+    @client.debug_dev = str = ''.dup
     assert_equal("delete", @client.delete(serverurl + 'servlet', param).content)
     assert_equal({'1' => ['2'], '3' => ['4']}, HTTP::Message.parse(str.split(/\r?\n\r?\n/)[2]))
   end
@@ -1309,14 +1310,14 @@ def test_post_body
   end
 
   def test_extra_headers
-    str = ""
+    str = "".dup
     @client.debug_dev = str
     @client.head(serverurl, nil, {"ABC" => "DEF"})
     lines = str.split(/(?:\r?\n)+/)
     assert_equal("= Request", lines[0])
     assert_match("ABC: DEF", lines[4])
     #
-    str = ""
+    str = "".dup
     @client.debug_dev = str
     @client.get(serverurl, nil, [["ABC", "DEF"], ["ABC", "DEF"]])
     lines = str.split(/(?:\r?\n)+/)
@@ -1326,7 +1327,7 @@ def test_extra_headers
   end
 
   def test_http_custom_date_header
-    @client.debug_dev = (str = "")
+    @client.debug_dev = (str = "".dup)
     _res = @client.get(serverurl + 'hello', :header => {'Date' => 'foo'})
     lines = str.split(/(?:\r?\n)+/)
     assert_equal('Date: foo', lines[4])
@@ -1416,9 +1417,9 @@ def test_cookies
   end
 
   def test_eof_error_length
-    io = StringIO.new('')
+    io = StringIO.new(''.dup)
     def io.gets(*arg)
-      @buf ||= ["HTTP/1.0 200 OK\n", "content-length: 123\n", "\n"]
+      @buf ||= ["HTTP/1.0 200 OK\n".dup, "content-length: 123\n".dup, "\n".dup]
       @buf.shift
     end
     def io.readpartial(size, buf)
@@ -1444,9 +1445,9 @@ def io.eof?
   end
 
   def test_eof_error_rest
-    io = StringIO.new('')
+    io = StringIO.new(''.dup)
     def io.gets(*arg)
-      @buf ||= ["HTTP/1.0 200 OK\n", "\n"]
+      @buf ||= ["HTTP/1.0 200 OK\n".dup, "\n".dup]
       @buf.shift
     end
     def io.readpartial(size, buf)
@@ -1870,7 +1871,7 @@ def test_charset
 
   if RUBY_VERSION >= "1.9.3"
     def test_continue
-      @client.debug_dev = str = ''
+      @client.debug_dev = str = ''.dup
       res = @client.get(serverurl + 'continue', :header => {:Expect => '100-continue'})
       assert_equal(200, res.status)
       assert_equal('done!', res.body)
diff --git a/test/test_include_client.rb b/test/test_include_client.rb
index 8cd05c46..10162b85 100644
--- a/test/test_include_client.rb
+++ b/test/test_include_client.rb
@@ -1,4 +1,5 @@
 # -*- encoding: utf-8 -*-
+# frozen_string_literal: true
 require File.expand_path('helper', File.dirname(__FILE__))
 
 require 'httpclient/include_client'
diff --git a/test/test_jsonclient.rb b/test/test_jsonclient.rb
index ff047be1..51462bbb 100644
--- a/test/test_jsonclient.rb
+++ b/test/test_jsonclient.rb
@@ -1,4 +1,5 @@
 # -*- encoding: utf-8 -*-
+# frozen_string_literal: true
 require File.expand_path('helper', File.dirname(__FILE__))
 require 'jsonclient'
 
diff --git a/test/test_ssl.rb b/test/test_ssl.rb
index a58ea822..8084e4a7 100644
--- a/test/test_ssl.rb
+++ b/test/test_ssl.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require File.expand_path('helper', File.dirname(__FILE__))
 require 'webrick/https'
 require 'time'
@@ -32,7 +33,7 @@ def test_proxy_ssl
       @client.ssl_config.set_client_cert_file(path('client.cert'), path('client.key'))
       @client.ssl_config.add_trust_ca(path('ca.cert'))
       @client.ssl_config.add_trust_ca(path('subca.cert'))
-      @client.debug_dev = str = ""
+      @client.debug_dev = str = "".dup
       assert_equal(200, @client.get(@url).status)
       assert(/accept/ =~ @proxyio.string, 'proxy is not used')
       assert(/Host: localhost:#{serverport}/ =~ str)
@@ -71,7 +72,7 @@ def test_sync
 end
 
   def test_debug_dev
-    str = @client.debug_dev = ''
+    str = @client.debug_dev = ''.dup
     cfg = @client.ssl_config
     cfg.client_cert = path("client.cert")
     cfg.client_key = path("client.key")
diff --git a/test/test_webagent-cookie.rb b/test/test_webagent-cookie.rb
index 29a415b4..7748a10f 100644
--- a/test/test_webagent-cookie.rb
+++ b/test/test_webagent-cookie.rb
@@ -1,3 +1,4 @@
+# frozen_string_literal: true
 require 'test/unit'
 require 'uri'
 require 'tempfile'