[FEATURE] ember-testing-checkbox-helpers

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 the `check` call or the `expectAssertion` call? 

I've wrapped the `check` call to no avail