Adding ability to mix can.event into can.Controls safely

When using the new `can/event` mixin, it can cause conflicts with can.Control-based classes due to duplicate methods `can.Control#on` and `can.Control#off`. (#981)

This syntax used to result in no event handlers being bound, but this pull request fixes that:
```javascript
var MyControl = can.Control.extend(can.extend({}, can.event, {
  " click": function() { /* didn't fire before, but does now */ }
}));
```