Fix estimator_spec failures

Looks like Rails 4 calls `marked_for_destruction?` when creating
records. And since all shipping method instances have their calculators
stubbed we also need to stub that method. This avoids errors like:

  Failure/Error: let(:shipping_methods) { 2.times.map { create(:shipping_method) } }
    Mock received unexpected message :marked_for_destruction? with (no args)
Just check if updated_at changed on asset_spec

That should be enough to reflect if touching a Spree::Asset would update
the viewable association