Get all attributes of Ember.Object

Let's say I have Ember model object:

App.ModelObject = Ember.Object.extend({
   attribute1: null,
   attribute2: 0,
   attribute3: "",

   init: function(){
       var self = this;
       self._super();
       console.log(Ember.keys(self).length);
   }
});

var obj = App.ModelObject.create(); //logs to console "0"

Than this happens (and I think it is undesired):

Ember.keys(obj).length; // === 0

obj.set("attribute1", 1);
Ember.keys(obj).length; // === 1

obj.set("attribute1", null);
Ember.keys(obj).length; // === 1

obj.set("attribute2", 0);
Ember.keys(obj).length; // === 1

obj.set("attribute2", null);
Ember.keys(obj).length; // === 2

What I have to do if I want to get all keys (attributes) of model, so it should return ["attribute1", "attribute2", "attribute3"]?