[BUGFIX] Ensure ArrayMixin#invoke returns an Ember.A.

[rwjblue]

This is a partial revert of bee179d, moving back to a manual forEach so that we can control the return value a bit more. When prototype extensions are disabled, we cannot rely on this.map to return an extended array.

Fixes a regression detected by ember-data's test suite running against ember-source's canary channel.

[runspired]

My only thought is that nothing outside of this one test in ember-data seems to have broken, and I haven't seen complaints from other folks (yet). The this.map behavior seems desirable. Do we maybe want to keep this fix in our pocket and just make ember-data not require the A for now?

[rwjblue]

@runspired - The issue is that this.map for an Ember.A() doesn't always return an Ember.A() (when prototype extensions are disabled). Changing what invoke returns definitely seems like a breaking change to me.

Also, its unlikely to have affected many ember-data users yet because its only on canary, but I'm certain that folks are doing "special" things in their buildURL where they expect the snapshots to be an extended thing. Think:

buildURL(type, ids, snapshots) {
  let first = snapshots.get('firstObject'); // :boom: without this change

  // ...snip...
}