Moved html_document to ActionDispatch::Assertions. Included the Rails::Dom::Testing::Assertions there as well.

actionpack/lib/action_controller/test_case.rb

@@ -684,14 +684,9 @@ module ActionController
       private
 
       def document_root_element
-        @html_document ||= if @response.content_type =~ /xml$/
+        html_document.root
-          Loofah.xml_document(@response.body)
-        else
-          Loofah.document(@response.body)
-        end.root
       end
 
-
       def check_required_ivars
         # Sanity check for required instance variables so we can give an
         # understandable error message.

actionpack/lib/action_dispatch/testing/assertions.rb

@@ -1,3 +1,6 @@
+require 'loofah'
+require 'rails-dom-testing'
+
 module ActionDispatch
   module Assertions
     autoload :ResponseAssertions, 'action_dispatch/testing/assertions/response'
@@ -7,6 +10,14 @@ module ActionDispatch
 
     include ResponseAssertions
     include RoutingAssertions
+    include Rails::Dom::Testing::Assertions
+
+    def html_document
+      @html_document ||= if @response.content_type =~ /xml$/
+        Loofah.xml_document(@response.body)
+      else
+        Loofah.document(@response.body)
+      end
+    end
   end
 end
-