Modifier keys cause highlighted autocomplete to search with currently highlighted option.

In an attempt to make it so that tab and shift-tab could cycle through the options in an autocomplete widget, I found that shift (and the other modifier keys) fall through the switch case that begins on line 108 (current Git version) in the keydown event handler, which causes the _searchTimeout function to be called when they are hit. This prevents them from being used to operate in conjunction with any other key without forcing a search. In the case of using shift-tab to cycle up through the options, when shift is pressed, the search is called, the first item is selected, and the autocomplete list shortens to items that begin with it before I can even hit tab. An easy fix is to simply expand the switch statement to ignore modifiers if they are the only key hit and leave them up to the programmer to handle on each specific input:

https://github.com/jquery/jquery-ui/compare/master...aiudirog:patch-1

However, I do not know if this is the proper fix or if it necessarily should be fixed, which is why I didn't submit a pull request.

Thank you