public function getEncoding()
    {
        $content = $this->getHeaderLine('content-type');
        if (!$content) {
            return null;
        }
        preg_match('/charset\s?=\s?[\'"]?([a-z0-9-_]+)[\'"]?/i', $content, $matches);
        if (empty($matches[1])) {
            return null;
        }

        return $matches[1];
    }