Manually assign notification_email in the User factory when stubbed

Because we assign this value in the model via a callback conditionally
on `email_changed?`, this never gets set when using `build_stubbed`,
resulting in a "can't be blank" validation error on this field.

In this case, we can just assign it manually to the same value as
`email`, which is generated via a sequence.

spec/factories/users.rb

@@ -8,6 +8,10 @@ FactoryGirl.define do
     confirmation_token { nil }
     can_create_group true
 
+    after(:stub) do |user|
+      user.notification_email = user.email
+    end
+
     before(:create) do |user|
       user.ensure_rss_token
     end