Fix AngularJS $q.when definition

When $q.when is called on a promise<T>, it returns a promise<T>, not a promise of a promise. The angular docs aren't totally clear on this, but it can be verified by checking out the [code](https://github.com/angular/angular.js/blob/master/src/ng/q.js) or running the following:

``` typescript
var foo = $q.when(1);
foo.then(x => console.log(x)); // logs 1
var bar = $q.when($q.when(2));
bar.then(x => console.log(x)); // logs 2
```