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

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.

Author: rwjblue

packages/ember-runtime/lib/mixins/array.js

@@ -951,7 +951,11 @@ const ArrayMixin = Mixin.create(Enumerable, {
     @public
   */
   invoke(methodName, ...args) {
-    return this.map(item => tryInvoke(item, methodName, args));
+    let ret = A();
+
+    this.forEach(item => ret.push(tryInvoke(item, methodName, args)));
+
+    return ret;
   },
 
   /**