Dummy app failures

[oliverlangan]

While adding tests to #62, I saw that the dummy app demo is broken. It fails with TypeError: undefined is not an object (evaluating 'this.args.model')

The initial issue appears to be within tests/dummy/app/controllers/application.js, which references this.args.model rather than this.model.

However, fixing that exposes a deeper problem: Error: Assertion Failed: You attempted to update isCheckedon(unknown object), but it had already been used previously in the same computation..