flush operation will overwrite the origin status code (#1460)

The status of responseWriter will be overwrite if flush was called. This is caused by the Flush of http.response.Flush().

response_writer.go

@@ -110,5 +110,6 @@ func (w *responseWriter) CloseNotify() <-chan bool {
 
 // Flush implements the http.Flush interface.
 func (w *responseWriter) Flush() {
+	w.WriteHeaderNow()
 	w.ResponseWriter.(http.Flusher).Flush()
 }