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>