public function options()
    {
        $request = new HTTP\Request('OPTIONS', $this->getAbsoluteUrl(''));
        $response = $this->send($request);

        $dav = $response->getHeader('Dav');
        if (!$dav) {
            return [];
        }

        $features = explode(',', $dav);
        foreach ($features as &$v) {
            $v = trim($v);
        }

        return $features;
    }