Handle Doctrine proxies correctly in PagerfantaHandler

JMSSerializerBundle has a [DoctrineProxySubscriber](https://github.com/schmittjoh/serializer/blob/master/src/JMS/Serializer/EventDispatcher/Subscriber/DoctrineProxySubscriber.php) which loads Doctrine Proxies before serializing them.

When I serialize a Pagerfanta object with, for example, 2 users. One user is a Proxy and 1 user is loaded completely. I've set `@XmlRoot("user")`. When I serialize the Pagerfanta object to XML one user has a `<entry>` root and one has a `<user>` root.

This PR fixes it.