Dont set @html_document to not collide with ActionDispatch

ActionDispatch Assertions define this var, and it will memoaize it
https://github.com/rails/rails/blob/master/actionpack/lib/action_dispatch/testing/assertions.rb#L15

Thus, we cannot define the same, otherwise they will collide.

Author: arthurnn

lib/rails/dom/testing/assertions/tag_assertions.rb

@@ -146,7 +146,7 @@ def _find_tag(conditions)
 
             def html_scanner_document
               xml = @response.content_type =~ /xml$/
-              @html_document ||= HTML::Document.new(@response.body, false, xml)
+              @html_scanner_document ||= HTML::Document.new(@response.body, false, xml)
             end
         end
       end