Fix ActiveModel::Conversion._to_partial_path not using a model's model_name.

The current implementation of _to_partial_path composes the part of two bits, `collection' and `element'.
ActiveModel::Name also contains these fields, and they are derived the same way _to_partial_path does it.
However, _to_partial_path doesn't use the information in model_name, and solely relies on its own computations instead.
This works for all standard cases, but not necessarily for models that provide a non-standard model_name.

This commit fixes that and has _to_partial_path use model_name if the class responds to it.

activemodel/lib/active_model/conversion.rb

@@ -108,7 +108,9 @@ module ActiveModel
       # Provide a class level cache for #to_partial_path. This is an
       # internal method and should not be accessed directly.
       def _to_partial_path # :nodoc:
-        @_to_partial_path ||= begin
+        @_to_partial_path ||= if respond_to?(:model_name)
+          "#{model_name.collection}/#{model_name.element}"
+        else
           element = ActiveSupport::Inflector.underscore(ActiveSupport::Inflector.demodulize(name))
           collection = ActiveSupport::Inflector.tableize(name)
           "#{collection}/#{element}"

activemodel/test/cases/conversion_test.rb

@@ -54,6 +54,10 @@ class ConversionTest < ActiveModel::TestCase
     assert_equal "helicopter/comanches/comanche", Helicopter::Comanche.new.to_partial_path
   end
 
+  test "to_partial_path handles non-standard model_name" do
+    assert_equal "attack_helicopters/ah-64", Helicopter::Apache.new.to_partial_path
+  end
+
   test "#to_param_delimiter allows redefining the delimiter used in #to_param" do
     old_delimiter = Contact.param_delimiter
     Contact.param_delimiter = "_"

activemodel/test/models/helicopter.rb

@@ -7,3 +7,16 @@ end
 class Helicopter::Comanche
   include ActiveModel::Conversion
 end
+
+class Helicopter::Apache
+  include ActiveModel::Conversion
+
+  class << self
+    def model_name
+      @model_name ||= ActiveModel::Name.new(self).tap do |model_name|
+        model_name.collection = "attack_helicopters"
+        model_name.element = "ah-64"
+      end
+    end
+  end
+end