[BUG] Transform `{NaN,Infitity,-Infinity}` to null

https://github.com/emberjs/data/pull/2509#discussion_r20898264

Serializing and Deserializing invalid numeric `JSON` (`NaN`,
`Infinity`, `-Infinity`) should result in `null`.
I'm checking for the value's validity.

```js
NaN === NaN // => false
1 === 1           // => true
```