Seralize belongsTo when type specified by string

Based this off @drogus comment in https://github.com/ebryn/ember-model/pull/113#issuecomment-20995481



The tricky thing is that if you have a bunch of materialized relationships, but those relationships are never accessed the Computed Property will never get calculated. This means you can't rely on any meta setting inside the computed property. You also can't do it outside the computed property, because its possible the type might not be defined yet.