Incorrect styling when there's only one button in a vertical button group

Created by: jedrichards

When there's only one button in a vertical button group its corner's aren't rounded properly. The top corners are still unrounded as if it were part of a button group. Ideally the below HTML should render a normal looking button with all 4 corners rounded.

<div class="btn-group btn-group-vertical">
    <button class="btn">Label</button>
</div>

Thx!