Use *_url instead of *_path to avoid a deprecation warning
HAX: check the `message` object, not the `MessageDelivery` object

See 669bf73 for background. It's probably better to rewrite these test without
using the internal `NullMail` class anyway.
Added back the `last_use` timestamp to the `AbstractAdapter`

This was removed in https://github.com/rails/rails/commit/9e457a8654fa89fe329719f88ae3679aefb21e56
Upgrade to the latest AMS 0.8.x (unreleased)

The current released version (0.8.2) does not work with Rails master at all. In
fact, it was quite surprising to me that this is the only test that broke...

See https://github.com/rails-api/active_model_serializers/pull/655