Add Test Coverage for `DS.NumberTransform`

Transforms are not tested at the `unit` level. Adding tests at the
`unit` granularity allows for regression testing if the implementation covered
by the `acceptance` level tests should change.
@stefanpenner @igorT this covers the case, but I'm not too keen on the approach.

This covers the current implementation. If I were to add:

```js
equal(transform.deserialize(NaN), null);
```

I would have to change the implementation