.dropdown-header padding-y should reference $dropdown-item-padding-y

Created by: k-funk

https://github.com/twbs/bootstrap/blob/master/scss/_dropdown.scss#L179

It seems to me that a .dropdown-header should use the y-padding of a dropdown-item ($dropdown-item-padding-y), not the dropdown-menu.

The problem I have is that I want to set $dropdown-padding-y: 0; for my menus, so that the items are the only thing controlling y-padding within the dropdown. However, this ends up effecting .dropdown-headers by putting 0-y padding on them as well.