Reset tracking before calling callback on model in save method

Model tracking should be reset before calling old `success` callback during save method. For now, this example from documentation doesn't work: 
```javascript
model.save({}, {
    success: function() {
        console.log(model.unsavedAttributes());  // >> false
    }
});
```
because we call success and then reset tracking. This PR solve this problem.

Thanks.