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, @fivetanley , then do we agree in `serialize:'excluded'`? 

Whenever, it is confirmed, I will update the PR and merge the different commits in an unique commit.