normalize values on newElement for radio and checkbox types - Fix #2538

fixes #2538

Browsers [behave different](https://github.com/mootools/mootools-core/issues/2538#issuecomment-38401336) regarding the value for a new created (and checked) `input` Element. 

This happens for types `radio` & `checkbox`. This PR fixes that and normalizes to the default specified in [W3 directives](http://www.w3.org/TR/html5/forms.html#dom-input-value) `'on'` when the value is not defined in the constructor.
Good point. That is more correct to what the W3 specs say:

>If the field element has a value attribute specified, then let value be the value of that attribute; otherwise, let value be the string "on"

Will update with 

    ... props.value == null) props.value = 'on';

to not override cases where value is falsy but has been defined.