[BUGFIX beta] Ember.Map#forEach callback should include map being iterated over

According to the ES6-spec, the callback for `forEach` on `Map` should include the Map being iterated over as the third parameter.

Source: https://people.mozilla.org/~jorendorff/es6-draft.html#sec-map.prototype.foreach
Merge pull request #9633 from kimroen/bugfix-ember-map-foreach

[BUGFIX beta] Ember.Map#forEach callback should include map being ite...