public function optionalParameters($params = null)
  {
    if (null === $params)
    {
      return $this->responseConfig['optionalParameters'];
    }

    if (false === is_array($params))
    {
      throw new InvalidArgumentException(sprintf(
        "%s is no valid parameter: Use an array with Key => Value Pairs", $params
      ));
    }

    $this->responseConfig['optionalParameters'] = $params;

    return $this;
  }