Merge pull request #2318 from rwcarlsen/fix-closenotify-panic2

fix another panic where CloseNotify was called from wrong goroutine

core/corehttp/gateway_handler.go

@@ -114,9 +114,10 @@ func (i *gatewayHandler) getOrHeadHandler(w http.ResponseWriter, r *http.Request
 	defer cancel()
 
 	if cn, ok := w.(http.CloseNotifier); ok {
+		clientGone := cn.CloseNotify()
 		go func() {
 			select {
-			case <-cn.CloseNotify():
+			case <-clientGone:
 			case <-ctx.Done():
 			}
 			cancel()