Fixed trouble when i tryied to validate something in context of BusyButton onClick

I found this bug in next case :

```javascript
require(["dojox/form/BusyButton", "dojo/on"], function (Button, on){
    var t = new Button({label: 'Avail', 
                        busyLabel: 'Busy'});
    t.placeAt(dojo.body());
    on(t, 'click', function (){
        // Invalid validation
        t.cancel();
    });
})
```