Test for extractable_options? within nested fields_for.

This fixes an error when a record object that is a subclass of Hash is
passed to fields_for, which is incorrectly interpreted as field options.

actionpack/lib/action_view/helpers/form_helper.rb

@@ -1218,7 +1218,7 @@ def #{selector}(method, options = {})  # def text_field(method, options = {})
       end
 
       def fields_for(record_name, record_object = nil, fields_options = {}, &block)
-        fields_options, record_object = record_object, nil if record_object.is_a?(Hash)
+        fields_options, record_object = record_object, nil if record_object.is_a?(Hash) && record_object.extractable_options?
         fields_options[:builder] ||= options[:builder]
         fields_options[:parent_builder] = self
 

actionpack/test/lib/controller/fake_models.rb

@@ -170,6 +170,17 @@ class Author < Comment
   def post_attributes=(attributes); end
 end
 
+class HashBackedAuthor < Hash
+  extend ActiveModel::Naming
+  include ActiveModel::Conversion
+
+  def persisted?; false; end
+
+  def name
+    "hash backed author"
+  end
+end
+
 module Blog
   def self._railtie
     self

actionpack/test/template/form_helper_test.rb

@@ -1564,6 +1564,22 @@ def test_nested_fields_uses_unique_indices_for_different_collection_associations
     assert_dom_equal expected, output_buffer
   end
 
+  def test_nested_fields_for_with_hash_like_model
+    @author = HashBackedAuthor.new
+
+    form_for(@post) do |f|
+      concat f.fields_for(:author, @author) { |af|
+        concat af.text_field(:name)
+      }
+    end
+
+    expected = whole_form('/posts/123', 'edit_post_123', 'edit_post', :method => 'put') do
+      '<input id="post_author_attributes_name" name="post[author_attributes][name]" size="30" type="text" value="hash backed author" />'
+    end
+
+    assert_dom_equal expected, output_buffer
+  end
+
   def test_fields_for
     output_buffer = fields_for(:post, @post) do |f|
       concat f.text_field(:title)