DragBoxInteraction: getGeometry return type can be ol.geom.Polygon ?

since this.box_ is ```ol.render.Box``` which has ```ol.geom.Polygon```, getGeometry on the DragBoxInteraction can be more specific, so ```ol.geom.Polygon``` instead of ```ol.geom.Geometry```

This will also make it easier for people doing a custom build to export the right things.