[BUGFIX release] `Ember.keys` should return own property

In IE8, `Ember.keys` couldn't return property that named the same as
own prototype property.
[BUGFIX release] `Ember.Object.extend` should allow any prorerty
Merge pull request #3508 from tricknotes/fix-object-extend-for-old-ie

[BUGFIX release] Fix Ember.Object.extend for old IE