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

[tricknotes]

In old IE, Ember.Object.extend couldn't extend property that is named the same as a property of Object.prototype.

MyClass = Ember.Object.extend({
  toString: function() {
    return 'MyClass'
  }
});

var object = MyClass.create();

// The following should return `'MyClass'`.
object.toString(); //=> '<(unknown mixin):ember123>'

The cause is that for ~ in loop doesn't enumerate a property of Object.prototype in old IE.
To fix this issue, I fixed Ember.keys to return its own properties and I used it for Ember.Object.extend.

[stefanpenner]

related: this tries to (potentially once did?) fix part of something similar https://github.com/emberjs/ember.js/blob/master/packages/ember-metal/lib/mixin.js#L325-L326

[tricknotes]

You are right! (but I have not do it yet.)

Should I try to apply this fix to similar points on this PR?