A DocumentModel added to a collection should be immediately aware of its collection membership

This was detected by NXMOB-238: the document returned by the Document.AddToFavorites operation was showing false for the isFavorite enricher although it had actually been added as a Favorite.

This was due to the use of a temporary DocumentModel in

CollectionManagerImpl#addToCollection(final DocumentModel collection, final DocumentModel documentToBeAdded, final CoreSession session)

that forced the need to re-fetch the document from the session once added to a collection to get a fresh state, which was done in the tests, but not in Document.AddToFavorites:

    public DocumentModel run(DocumentModel doc) {
        favoritesManager.addToFavorites(doc, session);
        return doc;
    }

The fix allows to remove these extra doc = session.getDocument(doc.getRef()) calls from the tests.

Note that the problem didn't happen when removing a document from a collection because there is no such use of a temporary DocumentModel in

CollectionManagerImpl#doRemoveFromCollection(DocumentModel documentToBeRemoved, String collectionId, CoreSession session)