Invisible border of input-group in inverse navbar

Created by: fredgate

When in a inverse navbar I add a input-group containing a input and an add-on, the border of the input is badly rendered while the border of the add-on is ok. If the navbar is not inverse, all is fine. Here is a jsfiddle showing the problem : https://jsfiddle.net/mw7ts10m/

The problem is with version alpha 5 (tested on firefox and IE11).

With the alpha 2 version, the borders was well rendered, as showed in this jsfiddle : https://jsfiddle.net/mw7ts10m/2/ It seems broken since alpha 3.