Fix CollectionView's itemEvents args

The handler used for CollectionView's itemEvents, per #838, includes a different set of arguments than one would expect when the events bubble up through the parent collection view.

As an example – the traditional way:
```
this.on('itemview:some:event', function() {
  // arguments[0] === itemview
  // and all other arguments will be forward on in order
});
```

Unfortunately, that's not the case using itemEvents:
``` 
itemEvents: {
  'some:event': function() {
    // arguments[0] === 'event string'
    // arguments[1] === itemview
    // and all other arguments will be forward on in order
  }
}
```

The PR tests and fixes this inconsistency.