code: private void sendHeartbeat(WebSocket websocket) {
          ObjectNode heartbeatPacket = JsonNodeFactory.instance.objectNode();
          heartbeatPacket.put("op", GatewayOpcode.HEARTBEAT.getCode());
          heartbeatPacket.put("d", lastSeq);
          WebSocketFrame heartbeatFrame = WebSocketFrame.createTextFrame(heartbeatPacket.toString());
          nextHeartbeatFrame.set(heartbeatFrame);
          websocket.sendFrame(heartbeatFrame);
      }
docs: Sends the heartbeat.
  
  @param websocket The websocket the heartbeat should be sent to.