public function getHeader($header, $first = true)
    {
        $normalizedHeader = str_replace('-', '_', strtolower($header));
        foreach ($this->headers as $key => $value) {
            if (str_replace('-', '_', strtolower($key)) === $normalizedHeader) {
                if ($first) {
                    return is_array($value) ? (count($value) ? $value[0] : '') : $value;
                }

                return is_array($value) ? $value : array($value);
            }
        }

        return $first ? null : array();
    }