Admin table CSS fix

Fixes admin tables that have a `<th>` tag in the `<tbody>` so that they have a top border:

**Before**

![iika](https://f.cloud.github.com/assets/4784/814855/badadb70-ef1d-11e2-9f15-8a283de0fb1d.png)

**After**

![b2jx](https://f.cloud.github.com/assets/4784/814857/c51bcfe0-ef1d-11e2-9fd4-e7bdf186f16a.png)