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.
Join the previous line and you'll get:

```js
if (props) {
  if (props.checked != null) props.defaultChecked = props.checked;
  if ((props.type == 'checkbox' || props.type == 'radio') && props.value == null) props.value = 'on';
}
```