fix the bug when choices=NULL in updateCheckboxGroupInput()

as reported at https://groups.google.com/forum/?pli=1#!topic/shiny-discuss/K7chwrMCvkU

Besides, I wonder why the `value` is not passed to `receiveMessage()` in shiny.js; it seems reasonable to me that `updateCheckboxGroupInput(session, 'foo', selected = 'bar')` should check the box `bar`. Same thing applies to the radio buttons.