private function parseRequestHeaders($raw_headers)
    {
        $request_headers = new CaseInsensitiveArray();
        list($first_line, $headers) = $this->parseHeaders($raw_headers);
        $request_headers['Request-Line'] = $first_line;
        foreach ($headers as $key => $value) {
            $request_headers[$key] = $value;
        }
        return $request_headers;
    }