fix updating of Documents with event subscribers

fix for https://github.com/doctrine/phpcr-odm/issues/526

* [ ] add a test that converts a value back and forth via postLoad/prePersist events
* [ ] squash commits
It doesn't cover the update bug, as the field not persisted was in the parent object, it covers just the second part.

It's enough to add a field to `SomeEntity` and to verify that the value is correctly persisted in the db.

Moreover we are not able to verify if the `SomeEntity` is staged for update or not, if we don't alter the `status` field.

With your previous proposal the Document was updated each time, because the first call to `computeChangeSet`detected a change for the field and the second too, but it didn't detect that it was the original value, so the field was marked for update. 

I don't know tho if it's that easy to test which fields are staged for update...