Fix email confirmation link not updating email (#6187)

A change introduced in #6125 prevents
`Devise::Models::Confirmable#confirm` from being called for existing
users, which in turn leads to `email` not being set to
`unconfirmed_email`, breaking email updates. This also adds a test
that would've caught this issue.

spec/controllers/auth/confirmations_controller_spec.rb

@@ -12,20 +12,40 @@ describe Auth::ConfirmationsController, type: :controller do
   end
 
   describe 'GET #show' do
-    let!(:user) { Fabricate(:user, confirmation_token: 'foobar', confirmed_at: nil) }
+    context 'when user is unconfirmed' do
+      let!(:user) { Fabricate(:user, confirmation_token: 'foobar', confirmed_at: nil) }
 
-    before do
-      allow(BootstrapTimelineWorker).to receive(:perform_async)
-      @request.env['devise.mapping'] = Devise.mappings[:user]
-      get :show, params: { confirmation_token: 'foobar' }
+      before do
+        allow(BootstrapTimelineWorker).to receive(:perform_async)
+        @request.env['devise.mapping'] = Devise.mappings[:user]
+        get :show, params: { confirmation_token: 'foobar' }
+      end
+
+      it 'redirects to login' do
+        expect(response).to redirect_to(new_user_session_path)
+      end
+
+      it 'queues up bootstrapping of home timeline' do
+        expect(BootstrapTimelineWorker).to have_received(:perform_async).with(user.account_id)
+      end
     end
 
-    it 'redirects to login' do
-      expect(response).to redirect_to(new_user_session_path)
-    end
+    context 'when user is updating email' do
+      let!(:user) { Fabricate(:user, confirmation_token: 'foobar', unconfirmed_email: 'new-email@example.com') }
 
-    it 'queues up bootstrapping of home timeline' do
-      expect(BootstrapTimelineWorker).to have_received(:perform_async).with(user.account_id)
+      before do
+        allow(BootstrapTimelineWorker).to receive(:perform_async)
+        @request.env['devise.mapping'] = Devise.mappings[:user]
+        get :show, params: { confirmation_token: 'foobar' }
+      end
+
+      it 'redirects to login' do
+        expect(response).to redirect_to(new_user_session_path)
+      end
+
+      it 'does not queue up bootstrapping of home timeline' do
+        expect(BootstrapTimelineWorker).to_not have_received(:perform_async)
+      end
     end
   end
 end