model.set('attr', null) does not make attribute unset

[anton-kotenko]

var Model = require('./');                                                                                                                                                            
var ModelClass = Model.inherit({                                                                                                                                                      
    attributes: {                                                                                                                                                                     
        id: Model.attributeTypes.Id                                                                                                                                                   
    }                                                                                                                                                                                 
});                                                                                                                                                                                   
var model = new ModelClass();                                                                                                                                                         
model.set('id', 1);                                                                                                                                                                   
console.log(model.get('id'), model.isSet('id'));   // => 1 true
model.set('id', null);                                                                                                                                                                                                                                                                                                                                                      
console.log(model.get('id'), model.isSet('id'));   // => null, true  ????????