Dropdown.prototype.keydown prevents focus and swallows some keystrokes in contenteditable elements

Created by: bo01ean

The click event will fall through and close dropdowns that contain contenteditables. The space character will fall through and be cancelled in contenteditable elements in dropdowns.

Issue: http://codepen.io/anon/pen/oxraRw

PR: https://github.com/twbs/bootstrap/pull/19950