Allow passing parameters to Doctrine entities when serializing

I have a use case where I need to be able to pass in some parameters to a SubscribingHandlerInterface which serializes Doctrine entities. Currently, the DoctrineProxySubscriber will remove these because of : 
```
        if ( ! $virtualType) {
            $event->setType(get_parent_class($object));
        }
```
in the onPreSerialize function (which will effectively throw away any parameters you have provided). This pull request should fix this.

BTW I also think this is some leftover code that is only needed for Symfony 2.0, at first I just commented it out and that worked as well on Symfony 2.1.