fix bug that displays only the field.label_tag when using HiddenInput widget

when using HiddenInput widget django admin hides the field but shows the field's label (see the attached image). this is a fix for this behaviour.
![screen shot 2013-12-09 at 6 00 08 pm](https://f.cloud.github.com/assets/30587/1708687/cb016318-6112-11e3-9ad2-5d2b1361a7bd.png)