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