public function isJson()
    {
        if (isset($this->isJson)) {
            return $this->isJson;
        }

        // check based on content header
        if (strpos($this->getHeader('content_type'), 'application/json') !== false) {
            $this->isJson = true;
        } else {
            // check based on content
            json_decode($this->content);
            $this->isJson = (json_last_error() == JSON_ERROR_NONE);
        }

        return $this->isJson;
    }