attrs is currently ignored.

One could fix this by calling the private this.normalizeUsingDeclaredMapping json serializer method before extracting the attributes, relationships. This is how the json-api serializer does it: https://github.com/emberjs/data/blob/v2.11.0/addon/serializers/json-api.js#L409

I think it should be part of the hal serializer.
If you're interested i could open a PR to implement it.