private function getHeader($key)
    {
        if (isset($this->requestHeader[$key])) {
            $header = $this->requestHeader[$key]['value'];

            unset($this->requestHeader[$key]);
        } else {
            if ($key === 'host') {
                $header = $this->getHost();
            } elseif ($key === 'accept') {
                $header = '*/*';
            } elseif ($key === 'user-agent') {
                $header = self::$userAgent;
            }
        }

        return $header;
    }