Serialize hasmany as ids by default

Changed the default serializer behavior to serialize a hasMany to an array of ids by default. This is definitely not a "must-merge" PR but I want to look into the option of at least providing a config option in the relation setup. IMHO is you define a relation to be bi-directional, it's unexpected behavior if the persistence lib assumes your API to be one-directional.

What do you think?