Better tests for AV::RecordIdentifier

This commit intends to clarify the scope of ActionView::RecordIdentifier
methods `dom_id` and `dom_class`.

Most of the current documentation comes from da257eb8 (7 years ago) when
the decoupling of ActionView, ActiveRecord and ActiveModel was not a concern.

Since then, steps have been taken to reach such decoupling.
Therefore I think it's important to show that ActionView::RecordIdentifier
**does not strictly depend on the ActiveRecord API**:
any class `Post` implementing `post.to_key` and `post.model_name.param_key`
will work.

This commit adds a test to prove that ActionView::RecordIdentifier methods
can also be used on objects that do not subclass ActiveRecord::Base.