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.
@ibolmo  had to do `if (props && (props.type == 'checkbox' ...` anyway, for cases of new elements with no props. 
Updated and re-tested the pull request.