Many to many and on match destroy in manage_relationship #859

I think on_match: :destroy in manage_relationship is incorrect for many_to_many.

Right now it destroys a join resource. Instead it should destroy both a join and a destination like other types of relationships do.

If one wants to destroy only a join resource then they should use :unrelate.

Destroy is implemented correctly for on_missing where :destroy destroys both and :unrelate only join one. (Though there is a mistake in documentation that states that it is possible to pass [:join, :keys] with a long form.) That's the only other case with support for :destroy so it is strange that they are inconsistent.