Replace subscription spec with a better spec

Replace the subscription spec with something that's more of a unit test.
New test doesn't instantiate ActiveRecord objects, doesn't rely on save,
and stubs Spree::Chimpy.enqueue.

Existing resubscribe examples were removed, since were stubbing
resubscribe.
Fix resubscribe and add specs

A user can now re-enable their subscription.

Also removed useless code:
* undefined @new_record
* redundant call to merge_vars_changed?
{,un,re}subscribe should always return true

ActiveRecord callbacks stop after any falsy values, and prevent saves.
don't use ActiveSupport.notifications as a queue