Update with normalize

I've done it considering [@bmac comment](https://github.com/emberjs/data/pull/2096#issuecomment-49619854)
Yeah, but unlike the others, in `normalizeUsingDeclaredMapping`, the normalization is done only against the serializer's attrs hash, so I think we don't care about the model here. If I'm wrong, that mean the implementation of   `normalizeUsingDeclaredMapping` should change to take the model attrs in account.