Bordered and simultaneously responsive table has not 2px header underline when is in responsive mode (under 772px)

Created by: janrenn

What is purpose of line .table-responsive > .table-bordered > thead > tr:last-child > th, https://github.com/twbs/bootstrap/blob/master/dist/css/bootstrap.css#L1630? It causes that in responsive mode (under 772px) the header of responsive table, if is simultaneously table-bordered, is not underlined with 2px line as it is in desktop mode.