error in save entity without all fields that is defined in model validate .

when you find entity model with fields parameter and this result don't include fields that has defined in model validate, you can't save it and you take validate error on not selected fields.
This could be refactored as follows:
````
if ($rules = $options['validate']) {
  if (is_array($rules) && $entity->exists()) {
    $rules = /* ...*/;
  }
  $events = /* */
  // ..
}
```