Update default-widgets.php fixing according W3C

The internals of `checked` (`__checked_selected_helper`) 
`uses (string) $helper === (string) $current`.
If compared to TRUE, it doesn't works.
So I suggest changing to 'on' that seem to be the value used W3C http://www.w3.org/TR/html401/interact/forms.html#h-17.2.1