normalize value on newElement for checkbox (IE fix)

The default value of a checkbox should be 'on' if not overwritten

(http://www.whatwg.org/specs/web-apps/current-work/#dom-input-value-default-on),
and IE is not following the specification on this one on element
creation. Mootools should catch it though, or add 'on' as default value
when creating a new checkbox.