"git archive" download support

This needs changes in GitLab and gitlab_git too.

See merge request !2

I solved the problem in cookbook-gitlab-test

Use our own header instead of User-Agent

This will still help routing in HTTP proxies, while also allowing
gitlab-rails to see the User-Agent.

See merge request !6

Forward Host header to the auth backend

The Host header is handled specially by the Go http library and
needs special processing in order to have it forwarded with the
other headers to the auth backend.

This allows the Host header received by the backend to be consistent with other 
requests not going through gitlab-git-http-server.


See merge request !5

The Host header is handled specially by the Go http library and
needs special processing in order to have it forwarded with the
other headers to the auth backend.

Support Kerberos authentication "final leg" as per RFC4559

This for the passwordless Kerberos token authentication to comply with the RFC.

See merge request !4

This is a breaking API change. Instead of parsing repo path from the request
in gitlab-git-http-server, we now expect the auth backend to tell us the full
path to the repository. This change is needed to handle API download requests.

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