Fix bugs in r59360, r59361, r59363

Fix bugs in r59360, r59361, r59363

* spellcheck is not a boolean attribute; it is an enumerated attribute
  whose possible values are "true" and "false".  If it were boolean, the
  permitted constructs would be <input spellcheck>, <input
  spellcheck="spellcheck">, and <input spellcheck="">, which would all
  set it true, and it would only be set to false if omitted entirely.
  (It would be boolean if HTML5 had invented it, but can't be for
  historical reasons.)
* spellcheck is valid on any HTML element, not just input, and so should
  be stripped on any element.

For reference, a table of all HTML5 attributes can be found at:

<http://www.whatwg.org/specs/web-apps/current-work/multipage/section-index.html#attributes-0>