Registers Ajax formats with the Request class.
public function onKernelRequest(GetResponseEvent $event) {
$request = $event
->getRequest();
$request
->setFormat('drupal_ajax', 'application/vnd.drupal-ajax');
$request
->setFormat('drupal_dialog', 'application/vnd.drupal-dialog');
$request
->setFormat('drupal_modal', 'application/vnd.drupal-modal');
}