code: public function requestJson($method, $url, $options=array()){
          $options['headers'][] = 'Content-Type: application/json';
          $options['headers'][] = 'Content-Length: ' . strlen($options['body']);
          $this->request($method, $url, $options);
      }
docs: request as body data is json
  if you want to request Json, set Content-Type and Length;