Added test to prove 3rd code block in Guides, section "Rendering a Template"

I've been struggling with getting my code to work with the Guides section in the title.  Here's the code in the Guides itself:

``` js
    App.PostsRoute = Ember.Route.extend({
      renderTemplate: function() {
        this.render({ controller: 'favoritePost' });
      }
    });
```
I finally wrote a test (which I've adding here) to prove my understanding of it, and it now works.  Perhaps this would round out the test suite a bit more; I didn't see any other tests for this case.  It would certainly have helped me to find this test before.

Please reject this PR if this is a duplicate.

This is related to another issue, #2027.