Fix wrong styling for column-header.button

This patch set the border-radius of column header button to 0px
so, this didnt show rounded anymore.

![Fix, image in #3962](https://cloud.githubusercontent.com/assets/2650479/5429131/bd3ecd8c-83c7-11e4-9aed-1dedfdaa10bb.png)

Fixes SL#3962