Wrong z-index on active button in buttons groups

[mlcdsscania]

Describe the bug
Active button groups get the wrong z-index (1, should be 0). This results in the active button moving above the c-header while scrolling.

To Reproduce
Create a button group in a corporate-ui page with a c-header
Set a button in the group to be active
Minimize the window enough to get a scroll-bar
Scroll button group towards the c-header

Expected behavior
The complete button group (including an active item) should go below the c-header

Version of Corporate-ui
Version 4.0.2

Desktop information:
Please complete the following information

• Chrome
• Version 74
• OS: Win 10

[mmexvr]

scania/scania-theme#140