code: function HeadersMessage(arg, options) {
    Message.call(this, options);
    this.BlockHeader = options.BlockHeader;
    this.command = 'headers';
    $.checkArgument(
      _.isUndefined(arg) || (Array.isArray(arg) && arg[0] instanceof this.BlockHeader),
      'First argument is expected to be an array of BlockHeader instances'
    );
    this.headers = arg;
  }
docs: Sent in response to a `getheaders` message. It contains information about
  block headers.
  @param {Array} arg - An array of BlockHeader instances
  @param {Object=} options
  @param {Array=} options.headers - array of block headers
  @param {Function} options.BlockHeader - a BlockHeader constructor
  @extends Message
  @constructor