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.
```js
equal(transform.serialize(NaN), NaN);
```

is failing. Probably because:

```js
Ember.isEqual(NaN, NaN) // =>false
```