<< is deprecated, we should be using .add

[wesleyeewong]

As specified in the ActiveModel::Errors class using << is deprecated, we should be using g.add instead.

https://github.com/rails/rails/blob/6-1-stable/activemodel/lib/active_model/errors.rb#L645-L651

I added the model_name method so the specs would pass, it is trying to resolve model_name somewhere, and fails b/c it's an anonymous class.