Add a failing test for the changed_by property

the changed_by property is not set on staff admin saving
Improve the failing test

since it was using the default generic moderator on Book, the
`auto_approve_for_staff` was set to `True` and the test was not failing
like it should have been
Fix the `changed_by` property affectation

little explanation why : if `auto_approve_for_staff` is set to False
then `save` is never called on the moderated object because the status
is neither approved or rejected. so we prefer explicit call in every
case