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.
@igort and i already bikeshedded this out from `exclude: true` to `serialize: false`. We decided to be consistent and use serialize. I'm not sure I understand the Mixin concern; later mixins should be able to override that.