[BUG] `fillIn` test helper should support checkbox

The `fillIn` test helper should be able to handle checking and unchecking an
`<input type="checkbox">`. The helper can infer the input type based on the
provided value. If the value is a boolean, the input is assumed to be a
checkbox. If not, the input is assumed to support text.
@rwjblue these tests are failing, both complaining that `Ember.assert` is never run, and raising an exception from the assertion.

Is there something I'm missing when expecting an assertion from an async helper?

![screen shot 2014-12-22 at 11 44 51 am](https://cloud.githubusercontent.com/assets/2575027/5527841/f89c7290-89cf-11e4-8a3a-f3a47d6e6a43.png)