private function assertHttpMethod($method) {
        if (!$this->assertString($method)) return false;
        $validHttpMethods = array(
            'GET', 'POST', 'PUT', 'DELETE', 'HEAD', 'OPTIONS', 'TRACE', 'CONNECT', 'PATCH', 'LINK', 'UNLINK'
        );
        if (!in_array($method, $validHttpMethods)) return false;
        return true;
    }