Select uses wrong border variable for height calculation

Created by: andrewbelcher

To get selects matching the size of text inputs, a calculation is done which includes the width of the border. However, it uses $border-width instead of $input-btn-border-width, which means if the two don't match, your selects have the wrong height.

This and the fix can be seen in the demonstration at http://codepen.io/andrewbelcher/pen/rjbwox.

I've got a PR to fix it - #21994. Apologies if I shouldn't have also opened an issue - wondered whether issues were the better place for reporting.