Improve regression test for #593 to check proxy state

Checking if the proxy is initialized and manually loading it is a more accurate test for hydration behavior in DocumentPersister::loadReferenceManyCollectionOwningSide().
Use target class' DocumentPersister when preparing referenceMany criteria

Closes #602
Merge pull request #607 from doctrine/gh607

Use target class' DocumentPersister when preparing referenceMany criteria