code: protected function formatJson($response)
      {
          $response->getHeaders()->set('Content-Type', 'application/json; charset=UTF-8');
          if ($response->data !== null) {
              $options = $this->encodeOptions;
              if ($this->prettyPrint) {
                  $options |= JSON_PRETTY_PRINT;
              }
              $response->content = Json::encode($response->data, $options);
          }
      }
docs: Formats response data in JSON format.
  @param Response $response