Update the defaults based on its own data and the request.
array $defaults the getRouteDefaults array:
array the modified defaults. Each enhancer MUST return the $defaults but may add or remove values
Overrides RouteEnhancerInterface::enhance
public function enhance(array $defaults, Request $request) {
  if (empty($defaults['_controller']) && !empty($defaults['_content'])) {
    $type = $this->negotiation
      ->getContentType($request);
    if (isset($this->types[$type])) {
      $defaults['_controller'] = $this->types[$type];
    }
  }
  return $defaults;
}