message: add override, to reduce clang warning
diff: @@ -81,7 +81,7 @@ public:
  void initiate(t_initiate_cb) override;
  bool initiate_close() override { return false; }
  const char* name() const override { return NAME; }
  - void send_msg(const json_array& j);
  + void send_msg(const json_array& j) override;
  private:
  static const int FRAME_MSG_LEN_MASK = 0x00FFFFFF;
  