Toggle combobox when pressing down arrow.

Support for keyboard-only navigation.
(tested on bootstrap 2.3)

Author: HazCod

js/bootstrap-combobox.js

@@ -351,6 +351,10 @@
   , keyup: function (e) {
       switch(e.keyCode) {
         case 40: // down arrow
+         if (!this.shown){
+           this.toggle();
+         }
+         break;
         case 39: // right arrow
         case 38: // up arrow
         case 37: // left arrow