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.
I think the result of the bikeshedding of #2106 was to normalize on serialize: false, and it was just a matter of getting around to updating DS.EmbeddedRecordsMixin