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

Signed-off-by: Eugenia Dellapenna <eugenia.dellapenna@gmail.com>