Add a true ActiveRecord integration test.

[smudge]

Currently, none of the tests rely on an actual database model, using only ActiveModel::Model classes instead.

To test against an ActiveRecord model, we need to set up this development dependency and helper code to spin up a test database (sqlite is fine). Typically this is accomplished using a "dummy" rails app at spec/dummy, and a spec/rails_helper.rb setup file that can be required in Rails-specific tests.