When calling UnitOfWork#recomputeSingleEntityChangeSet on an entity
that didn't have a changeset before, the computation was ignored.
This method however is suggested to be used in "onFlush" and "preFlush"
events in the documentation.
Also fix a bug where recomputeSingleEntityChangeSet was used
before calculating a real changeset for an object.
The fix for DDC-2624 had a side effect on recomputation of
changesets in preUpdate events. The method wasn't adjusted
to the changes in its sister method computeChangeSet() and
had wrong assumptions about the computation.
Especially:
1. Collections have to be skipped
2. Comparison was changed to strict equality only.
