I've noticed that the contextual delete method is always returning 0 when write concern is set to 0.

Destroy on the other hand, correctly accounts the number of destroyed records with write concern set to either value (1, 0).

Both of those methods specify the return value as `nil` in their respective comments.
@return [ nil ] Nil.

Bisecting the issue reveals that this behavior affects all versions >= 5.2.0.
https://github.com/mongodb/mongoid/commit/c24acdb0c8ce52591176bd7af6bb454025628c58