Send User-Agent HTTP header on requests

[oznogon]

Some websites, such as GitHub's API, refuse requests from clients that don't provide a User-Agent header, so SP should pass one.

Not sure how to get a version number or specific app name here, but for example just this satisfies basic requirements:

index 789070a..4450f30 100644
--- a/src/io/http/request.cpp
+++ b/src/io/http/request.cpp
@@ -12,6 +12,7 @@ Request::Request(const string& host, int port, Scheme scheme)
 : port(port), scheme(scheme)
 {
     headers["Host"] = host;
+    headers["User-Agent"] = "SeriousProton/0.0.0";
 }
 
 void Request::setHeader(const string& key, const string& value)