code: function _write (chunk, encoding, callback) {
    /* istanbul ignore else */
    if (Array.isArray(this.options.input.buffer)) {
      /* istanbul ignore else */
      if (is.buffer(chunk)) {
        if (this.options.input.buffer.length === 0) {
          const that = this;
          this.on('finish', function () {
            that.streamInFinished = true;
          });
        }
        this.options.input.buffer.push(chunk);
        callback();
      } else {
        callback(new Error('Non-Buffer data on Writable Stream'));
      }
    } else {
      callback(new Error('Unexpected data on Writable Stream'));
    }
  }
docs: Handle incoming Buffer chunk on Writable Stream.
  @private
  @param {Buffer} chunk
  @param {String} encoding - unused
  @param {Function} callback