BB-867: Fix Author-Credit enabled checkbox

[MonkeyDo]

From https://tickets.metabrainz.org/projects/BB/issues/BB-867

While creating this edition I checked the "the does not have an author credit"-box (it is a book with many different authors) however, the checkbox does not persist, it is unchecked when I attempt to edit the edition, and must be-rechecked every time to allow other changes.
looking at the revision history also seems to not include any such changes.
Additionally: the edition page prompts you to add an author credit (even though it it was created with the check in place)

Simple solution: Force authorCreditEnable as a boolean, both when loading from DB and when creating/updating an edition.

[MonkeyDo]

That didn't work...