Fix serializer not using attrs before extracting attributes

[makepanic]

fixes #45

[bschouwerwou]

Thanks for this! I've been trying this out and it seems it doesn't handle nested embedded resources.

For example, given the following payload:

{
  _embedded: {
    item: [
      {
        _embedded: { foo: {...}, _links: {...} },
        _links: {...}
      }
    ]
  },
  _links: {...}
}

If I have defined attrs for the model foo, they are not used.