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.
Would any point the tag not be input (for the case of type is checkbox or type). You also prob. shouldn't care if props.value is own property. I'd defer for someone to correct me, though. 

Simplifying to:
``` js
if ((props.type == 'checkbox' || props.type == 'radio') && !props.value) props.value = 'on';
```