Note to self, @yihui, and @trestletech that with this change, ui.R can return two different content bodies depending on the URL parameters. This may have interactions with the Shiny Server static caching scheme we were thinking of, in conjunction with bookmarkable state (assuming that the latter will use URL parameters, not hash). Specifically, this change forces us to consider URL parameters to be part of the "cache key", which we might otherwise have been tempted not to do.