update the object state when calling PreUpdateEventArgs::setNewValue()

related to http://www.doctrine-project.org/jira/browse/DDC-3248 (guess ORM would then also need to be updated). or maybe it should only be done in specialized classes.
would be nice of course to have a dedicated method for this .. or we should keep things as is and simply add this logic in the ORM/ODM specific implementations of this class.