code: function PongMessage(arg, options) {
    Message.call(this, options);
    this.command = 'pong';
    $.checkArgument(
      _.isUndefined(arg) || (BufferUtil.isBuffer(arg) && arg.length === 8),
      'First argument is expected to be an 8 byte buffer'
    );
    this.nonce = arg || utils.getNonce();
  }
docs: A message in response to a ping message.
  @param {Number} arg - A nonce for the Pong message
  @param {Object=} options
  @extends Message
  @constructor