Make sure we close HTTP auth response bodies

We were seeing a resource leak:

2015/08/07 12:09:17 Get http://localhost:8080/gitlab/omnibus-gitlab.git/info/refs?service=git-upload-pack: dial tcp: lookup localhost: too many open files
2015/08/07 12:09:18 http: Accept error: accept tcp 127.0.0.1:8181: too many open files; retrying in 5ms

My guess is it was about not closing the auth response body.

main.go

@@ -125,6 +125,8 @@ func gitHandler(w http.ResponseWriter, r *http.Request) {
 		fail500(w, err)
 		return
 	}
+	defer authResponse.Body.Close()
+
 	if authResponse.StatusCode != 200 {
 		// The Git request is not allowed by the backend. Maybe the
 		// client needs to send HTTP Basic credentials.  Forward the