Binding to an Observe.compute value is broken

```js
var person = new can.Observe({
    name: 'Jeeves'
});
var name = person.compute('name');
name.bind('change', function(ev, newVal, oldVal) {
    ...
});
name('Alfred');
```

The change handler in this example will be called twice. Once with the `newVal` as `Alfred` and a second time with `newVal` as the Observe instance (`person`).

__Here's how this happens:__

When the `name` compute is updated, we end up in [compute.js](https://github.com/bitovi/canjs/blob/master/observe/compute/compute.js#L342) and run the `set` function which is defined here in [observe.js](https://github.com/bitovi/canjs/blob/master/observe/observe.js#L919). This calls `attr` which fires a change event with `newVal === 'Alfred'`. 

`setVal` is now the Observe instance since [attr returns the Observe](https://github.com/bitovi/canjs/blob/master/observe/observe.js#L538), so when the execution reaches [here](https://github.com/bitovi/canjs/blob/master/observe/compute/compute.js#L353), a change event is triggered with `newVal` set to the Observe instance.

This code was added by @justinbmeyer [here](https://github.com/bitovi/canjs/commit/e73635f150744cb77f5b0bd7ce898942894cd47d) to extend can.compute.