Add URL scheme param to run-server

Enables running proxy server against local Gerrit one like this:
./polygerrit-ui/run-server.sh --host=localhost:8080 --scheme=http

Change-Id: Ie35b11879751b1bb2d353d5a5849e4dfe443c4b3

polygerrit-ui/server.go

@@ -32,6 +32,7 @@ var (
 	restHost = flag.String("host", "gerrit-review.googlesource.com", "Host to proxy requests to")
 	port     = flag.String("port", ":8081", "Port to serve HTTP requests on")
 	prod     = flag.Bool("prod", false, "Serve production assets")
+	scheme   = flag.String("scheme", "https", "URL scheme")
 )
 
 func main() {
@@ -57,7 +58,7 @@ func handleRESTProxy(w http.ResponseWriter, r *http.Request) {
 	req := &http.Request{
 		Method: "GET",
 		URL: &url.URL{
-			Scheme:   "https",
+			Scheme:   *scheme,
 			Host:     *restHost,
 			Opaque:   r.URL.EscapedPath(),
 			RawQuery: r.URL.RawQuery,