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.
The `Sheep` fixture was added in eb23754e when moving template tests
from actionpack to actionview, but it's not actually used in ActionView tests.
The `Sheep` fixture is only used to test `uncountable` in ActiveModel tests,
and is already defined in activemodel/test/models/sheep.rb
