function buildKeyDownEvent (keyCode, eventProps) {
  return $.Event('keydown', $.extend({}, { which: keyCode }, eventProps));
}
Build a keydown event with the given key code and extra options.

@param  {Number} keyCode     the keyboard code to be used for the 'which'
                             attribute of the keydown event.
@param  {Object} eventProps  extra properties to build the keydown event.

@return {jQuery.Event} a 'keydown' type event.