protected function normalize($payload)
    {
        $data = get_object_vars($payload);
        foreach ($data as $k => $v) {
            if ($v instanceof \DateTime) {
                $payload->$k = $v->format('c');
                continue;
            }
            if (is_object($v)) {
                $payload->$k = $this->normalize($v);
                continue;
            }
        }
        return $payload;
    }