ngClass remove repeating classes

ngClass remove repeating classes when watched value is changed.
For example:

```
<span ng-class="{'same first': value, 'same second': !value}"></span>  
```

```
  $scope.value = true;

  $timeout(function () {
    $scope.value = false;
  }, 3000);
```

When value will be equal false class contains only "second"

```
<span class="second"></span>
```

This bug appear in 1.2.16 version.
