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.

githandler.go

@@ -149,6 +149,10 @@ func (h *gitHandler) doAuthRequest(r *http.Request) (result *http.Response, err
 	for k, v := range r.Header {
 		authReq.Header[k] = v
 	}
+	// Also forward the Host header, which is excluded from the Header map by the http libary.
+	// This allows the Host header received by the backend to be consistent with other
+	// requests not going through gitlab-git-http-server.
+	authReq.Host = r.Host
 	// 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")