Custom select border radius has default browser value when $enable-rounded is set to false

Created by: wojtekmieszczak

I'm using Bootstrap v4.0.0-alpha.6 on OS X 10.2.2.

Setting $enable-rounded to false instead of zeroing the value of border radius it allows to use browser default. However it works as expected when $enable-rounded is set to true and $custom-select-border-radius is set to 0.