Refactor registrant change mailer

#480

spec/services/registrant_change_spec.rb

@@ -0,0 +1,20 @@
+require 'rails_helper'
+
+RSpec.describe RegistrantChange do
+  describe '#confirm' do
+    let(:domain) { instance_double(Domain) }
+    let(:old_registrant) { instance_double(Registrant) }
+    let(:message) { instance_spy(ActionMailer::MessageDelivery) }
+
+    before :example do
+      allow(RegistrantChangeMailer).to receive(:confirmed)
+                                           .with(domain: domain, old_registrant: old_registrant)
+                                           .and_return(message)
+      described_class.new(domain: domain, old_registrant: old_registrant).confirm
+    end
+
+    it 'notifies registrant' do
+      expect(message).to have_received(:deliver_now)
+    end
+  end
+end