public function newRequest($method, $uri, $contentType = null, $body = null)
    {
        // TODO $uri whitelist (trusted domain)

        $headers = [
            'User-Agent' => $this->userAgent,
        ];
        if (!is_null($contentType)) {
            $headers['Content-Type'] = $contentType;
        }

        return new Request(
            $method,
            $uri,
            $headers,
            $body
        );
    }