protected function normalizeCustomHeaders()
    {
        $headers = [];

        // The spec requires that headers are sorted by header name.
        sort($this->headers);
        foreach ($this->headers as $header) {
            if ($this->request->hasHeader($header)) {
                $headers[] = strtolower($header) . ':' . $this->request->getHeaderLine($header);
            }
        }

        return $headers;
    }