[WIP] Add test to ensure document changeset gets overwritten after events

This test failed before #305.

Todo:
- [ ] Add failing tests for reorderings #306

Uncomment line https://github.com/doctrine/phpcr-odm/blob/8fc6074e5b937d3514b323a4ad2f307026ea35ec/lib/Doctrine/ODM/PHPCR/UnitOfWork.php#L1219 to see that it failed.