Make proxy objects serializable

When trying to serialize OGM results (proxy objects) with JMS Serializer, I got:

```
Resources are not supported in serialized data. Path: Everyman\\Neo4j\\Transport\\Curl -> Everyman\\Neo4j\\Client -> Everyman\\Neo4j\\Node -> neo4jProxyMyProxy -> Doctrine\\Common\\Collections\\ArrayCollection
```

JMS Serializer’s [DoctrineProxySubscriber](https://github.com/schmittjoh/serializer/blob/master/src/JMS/Serializer/EventDispatcher/Subscriber/DoctrineProxySubscriber.php) takes care of serializing Doctrine proxies, but didn't act on the OGM's proxies because they don't implement the `Doctrine\Common\Persistence\Proxy` interface. This PR fixes that, so OGM proxies will be serialized correctly.