public function post($command, $params = null, $postcontenttype = 'application/json')
    {
        $url = $command;

        $headers = array('Content-Type' => $postcontenttype);

        $content = $params;
        if ($postcontenttype == 'application/json') {
            $content = json_encode($params);
        }

        try {
            $response = $this->client->post($url, ['headers' => $headers, 'body' => $content]);
        } catch (\Exception $e) {
            return null;
        }
        $this->checkError($response, $url);

        return $response->getBody();
    }