Fixed embedded belongsTo relationships

This fixes an an error where the code

    Person = DS.Model.extend({
      name: DS.attr('string')
    });
    Group = DS.Model.extend({
      name: DS.attr('string'),
      people: DS.hasMany(Person)
    });
    Person.reopen({
      group: DS.belongsTo(Group)
    });
    Adapter.map(Person, {
      group: { embedded: 'always' }
    });

    store.createRecord(Person, { name: "Tom Dale", group: store.createRecord(Group, { name: "Ember.js Group" }) });
    store.commit();

would trigger 2 POST requests (1 to /people, 1 to /groups) where you would only expect 1 (because of group being embedded in person).