public function post($url, $data, $contentType = 'application/json')
    {
        $uri = $this->buildUrl($url);

        $request = $this->requestFactory->createRequest('POST', $uri);
        if (!is_null($contentType)) {
            $request = $request->withHeader('Content-Type', $contentType);
            $request = $request->withHeader('Accept', $contentType);
        }
        $request = $request->withBody(stream_for($data));

        $response = $this->request($request);

        return strval($response->getBody());
    }