Fixes `updated` callback to can.compute's on/off to simply need to be called.

The following was not working:

```js
var value = can.compute('', {
	get: function () {
		return input.value;
	},
	set: function (newVal) {
		input.value = newVal;
	},
	on: function (update) {
		input.onchange = update;
	},
	off: function () {
		delete input.onchange;
	}
});
```

If the user changed the input value, update would be called with the event, setting the value of the event to the compute.  This changes the update function to use the get function to retrieve a new value and check if the value has changed before triggering the event.