Dropdown: Change .divider to .disabled on keydown event.

[vsn4ik]

bootstrap/js/dropdown.js

Line 75 in ac49a09

var desc = ' li:not(.divider):visible a'

1. .divider never includes a.
2. I believe it is necessary to replace .divider to .disabled.

Current behavior: http://getbootstrap.com/components/#dropdowns-disabled.
Pure select skips disabled option: http://jsbin.com/demeqizefi/1/edit?html,output.

[ankurkaushal]

@cvrebert Should I go ahead and submit and pull request for it? @vsn4ik's suggestion works and it was quite a minor fix.