fix Helpers modified given helper module

Author: sevaorlov

CHANGELOG.md

@@ -13,6 +13,7 @@
 * [#1326](https://github.com/ruby-grape/grape/pull/1326): Fix wrong behavior for OPTIONS and HEAD requests with catch-all - [@ekampp](https://github.com/ekampp), [@namusyaka](https://github.com/namusyaka).
 * [#1330](https://github.com/ruby-grape/grape/pull/1330): Add `register` keyword for adding customized parsers and formatters - [@namusyaka](https://github.com/namusyaka).
 * [#1336](https://github.com/ruby-grape/grape/pull/1336): Do not modify Hash argument to `error!` - [@tjwp](https://github.com/tjwp).
+* [#1338](https://github.com/ruby-grape/grape/pull/1338): Fix helpers method modified given helper module - [@vsorlov](https://github.com/vsorlov).
 
 0.15.0 (3/8/2016)
 =================

lib/grape/dsl/helpers.rb

@@ -27,17 +27,21 @@ module ClassMethods
         #     end
         #
         def helpers(new_mod = nil, &block)
+          mod = Module.new
           if block_given? || new_mod
-            mod = new_mod || Module.new
+            if new_mod
+              define_boolean_in_mod new_mod
+              inject_api_helpers_to_mod new_mod
+              mod.send :include, new_mod
+            end
+
             define_boolean_in_mod(mod)
-            inject_api_helpers_to_mod(mod) if new_mod
             inject_api_helpers_to_mod(mod) do
               mod.class_eval(&block)
             end if block_given?
 
             namespace_stackable(:helpers, mod)
           else
-            mod = Module.new
             namespace_stackable(:helpers).each do |mod_to_include|
               mod.send :include, mod_to_include
             end

spec/grape/dsl/helpers_spec.rb

@@ -46,7 +46,8 @@ def test
           expect(subject).to receive(:namespace_stackable).with(:helpers).and_call_original
           subject.helpers(mod, &proc)
 
-          expect(subject.mod).to eq mod
+          expect(mod.instance_methods).not_to include :test
+          expect(subject.mod.instance_methods).to include :test
         end
 
         context 'with an external file' do