Tooltip - Hide not possible

```
<span slider ng-model="element.ngModel.alpha" min="0" step="0.1" max="1" tooltip="hide"></span>
```

Produces:
<img width="231" alt="bildschirmfoto 2016-01-26 um 19 45 44" src="https://cloud.githubusercontent.com/assets/2657649/12590865/6b285baa-c465-11e5-97f6-b1385bacdb98.png">
