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.

actionview/test/lib/controller/fake_models.rb

@@ -170,3 +170,15 @@ end
 
 class Car < Struct.new(:color)
 end
+
+class Plane
+  attr_reader :to_key
+
+  def model_name
+    OpenStruct.new param_key: 'airplane'
+  end
+
+  def save
+    @to_key = [1]
+  end
+end

actionview/test/template/record_identifier_test.rb

@@ -46,3 +46,46 @@ class RecordIdentifierTest < ActiveSupport::TestCase
     assert_equal @singular, ActionView::RecordIdentifier.dom_class(@record)
   end
 end
+
+class RecordIdentifierWithoutActiveModelTest < ActiveSupport::TestCase
+  include ActionView::RecordIdentifier
+
+  def setup
+    @record = Plane.new
+  end
+
+  def test_dom_id_with_new_record
+    assert_equal "new_airplane", dom_id(@record)
+  end
+
+  def test_dom_id_with_new_record_and_prefix
+    assert_equal "custom_prefix_airplane", dom_id(@record, :custom_prefix)
+  end
+
+  def test_dom_id_with_saved_record
+    @record.save
+    assert_equal "airplane_1", dom_id(@record)
+  end
+
+  def test_dom_id_with_prefix
+    @record.save
+    assert_equal "edit_airplane_1", dom_id(@record, :edit)
+  end
+
+  def test_dom_class
+    assert_equal 'airplane', dom_class(@record)
+  end
+
+  def test_dom_class_with_prefix
+    assert_equal "custom_prefix_airplane", dom_class(@record, :custom_prefix)
+  end
+
+  def test_dom_id_as_singleton_method
+    @record.save
+    assert_equal "airplane_1", ActionView::RecordIdentifier.dom_id(@record)
+  end
+
+  def test_dom_class_as_singleton_method
+    assert_equal 'airplane', ActionView::RecordIdentifier.dom_class(@record)
+  end
+end