Add some failing test when asserting body class

This is the reason that @rubys suite is broken.

As we are always using `Nokogiri::HTML::DocumentFragment` to parse the result [it add a body tag](https://github.com/sparklemotion/nokogiri/blob/62e8e5bb014654fe323e8a5e2a8e27a73871a30a/lib/nokogiri/html/document_fragment.rb#L33) and our body tag is ignored. So we can't assert the class of body.

If we always use fragment we can't fix https://github.com/rails/rails-dom-testing/issues/19.