docs: public void forward(String url, Object... args) {
          E.illegalArgumentIfNot(url.startsWith("/"), "forward URL must starts with single '/'");
          E.illegalArgumentIf(url.startsWith("//"), "forward URL must starts with single `/`");
          E.unexpectedIfNot(H.Method.GET == req().method(), "forward only support on HTTP GET request");
          uploads.clear();
          extraParams.clear();
          bodyParams = null;
          String target = S.fmt(url, args);
          urlPath = UrlPath.of(target);
          UndertowRequest req = $.cast(req());
          state = State.CREATED;
          req.forward(target);
          final RequestHandler requestHandler = router.getInvoker(H.Method.GET, url, this);
          requestHandler.handle(this);
      }