Add (failing) test case for <option selected="">

The selected attribute of the option should be true according to
  HTML4: http://www.w3.org/TR/REC-html40/intro/sgmltut.html#h-3.3.4.2 as well as
  HTML5: http://www.whatwg.org/specs/web-apps/current-work/#boolean-attributes
Fix selected attribute

The element <option selected=""> is selected.