Add support for JSONSerializer.attrs.serialize for relationships

Currently, the `attrs.serialize` property does not work for `hasMany` and `belongsTo` relationships, so relationships are always serialized.

This PR supports the `serialize` property on relationships.
@pixelhandler, I thought, it was  documented [here](https://github.com/emberjs/data/blob/master/packages/ember-data/lib/serializers/json_serializer.js#L71)?