Fix form_for to work with objects that implement to_model

Previously, if you tried to use form_for with a presenter object
that implements to_model, it would crash in
action_dispatch/routing/polymorphic_routes.rb when asking the presenter
whether it is .persisted?

Now, we always ask .persisted? of the to_model object instead.

This seems to been an issue since 1606fc9d840da869a60213bc889da6fcf1fdc431
@rafaelfranca The ModelDelegator (and ModelDelegate) was only used in a single existing test, and it seems more realistic to me that the object with to_model defined would not itself be an ActiveModel.

We could have both tests (where the delegator is a model vs not a model) if it seems appropriate, but to me this is the more valuable of the two.

I also chose here to make the ModelDelegate simply respond `true` to persisted? rather than making it an `ActiveRecord` object (which seems like a more common case), but could definitely go either way.