habtm#delete saves between the before and after hook.

When a has_and_belongs_to_many#delete method is called, the
relationships should be saved before the after_remove hook is called.
This enables the far side of the association to have the new
relationships in the after_remove hook.
Merge pull request #104 from projecthydra/save_between_hooks

habtm#delete saves between the before and after hook.