Add document_root_element to ActionDispatch::IntegrationTest so assert_select...

Add document_root_element to ActionDispatch::IntegrationTest so assert_select can be called without specifying a root.

actionpack/lib/action_dispatch/testing/integration.rb

@@ -493,5 +493,9 @@ def url_options
       reset! unless integration_session
       integration_session.url_options
     end
+
+    def document_root_element
+      html_document.root
+    end
   end
 end