BelongsToMany sync does't use configured foreign key

[Jérôme Tamarelle]

I guess there is an other issue with this call. It should be $ids = $this->parseIds($ids); also. Otherwise it's getting the model _id instead of the relatedKey.
Do you want to add an other test with a customized relatedKey? That will require a new model class for tests, but will ensure we use the correct ID field.

Originally posted by @GromNaN in #2648 (comment)