v4: change date() value before triggering 'change.dp' event

If you change the date from 'unset' state, `date()` returns null inside 'change.dp' handler. Yes, `e.date` has a correct value, but `date()` should be the same. And I think `e.oldDate` should be equal to null if the date was unset before. Demo: http://jsfiddle.net/3Lws5upg/2/
Also if you delete the text in input field using the keyboard, 'change.dp' event isn't triggerred at all. You can see this in the same demo.