app/controllers/omniauth_callbacks_controller.rb: swaps out the straight redirect request so that the page the user first chose is loaded after sign in.
spec/controllers/omniauth_callbacks_controller_spec.rb: brings in the tests from spec/requests/omniauth_callbacks_requests_spec.rb and adds new tests for the callback redirect.
spec/rails_helper.rb: enables the Warden testing helpers needed to make post :shibboleth work.
spec/requests/omniauth_callbacks_requests_spec.rb: removes it from testing.
post :shibboleth
work.