ol.interaction.DragBox.getGeometry returns non-closed polygon

Reproduce:
```javascript
var box = new ol.interaction.DragBox();
box.on('boxend',function(){
  console.log(box.getGeometry().getCoordinates());
});
```
It output array of 4 pairs of coordinates, that indicate all 4 corner points, but it should contain also 5th pair of coordinates that is equal to first pair of coordinates.