Add inverse of add target

Follow up with fix for https://github.com/rails/rails/pull/12413.

Add back `set_inverse_instance` on `.add_to_target`
We must have it in there too, so when an existent record is being concat to another, we will have the inverse relation.

This line is there on master. So we should add it back.
https://github.com/rails/rails/blob/master/activerecord/lib/active_record/associations/collection_association.rb#L378

review @rafaelfranca