Emails should deliver to nobody when perform_deliveries is set to false

The [Rails Guides](http://guides.rubyonrails.org/action_mailer_basics.html#action-mailer-callbacks) now recommend using an `after_filter` and setting `perform_deliveries` to
`false` as a method of preventing delivery. It's reasonable to expect that
`expect(email).not_to deliver_to(guest_user)` would work in this
scenario.