Added support for old way of embedding ids, without relation_ids = [1, 2].

Set ActiveModel::Serializer::Associations::HasMany.embed_ids_old_style
to true to active. For instance it will serialize comment ids to
comments = [1, 2, 3] instead of new way comment_ids = [1, 2, 3].

We need this in an legacy API of ours, and it needs to be done globally
for now :-(