public function authHeaders(array $headers = [ ])
    {
        if (count($headers) == 0) {
            $headers = [ 'X-Application' => Auth::$appKey, 'X-Authentication' => Auth::$sessionToken ];
        }
        $this->options[ 'headers' ] = array_merge($this->options[ 'headers' ], $headers);
        return $this;
    }