Merge branch 'send-user-agent' into 'master'

Add custom "User-Agent" header to auth requests

This makes it a lot easier to proxy requests to gitlab-git-http-server when Gitlab is running inside Passenger (or similiar) and therefore does not have a dedicated port.

Example Apache config:
~~~~
RewriteEngine on
RewriteCond "%{REQUEST_URI}" "^[-/\w\.]+\.git/"
RewriteCond "%{HTTP_USER_AGENT}" "!=gitlab-git-http-server"
RewriteRule . http://127.0.0.1:8181%{REQUEST_URI} [P,QSA]
~~~~

See merge request !1

githandler.go

@@ -140,6 +140,9 @@ func (h *gitHandler) doAuthRequest(r *http.Request) (result *http.Response, err
 	for k, v := range r.Header {
 		authReq.Header[k] = v
 	}
+	// Set custom user agent for the request. This can be used in some
+	// configurations (Passenger) to solve auth request routing problems.
+	authReq.Header.Set("User-Agent", "gitlab-git-http-server")
 	return h.httpClient.Do(authReq)
 }