Only remove /websocket/ from path if it's present

Today, when rmarkdown is serving a directory of Shiny content, it presumes that request URI will end with `/websocket/`. This is in fact the case in Shiny Server and Shiny run locally, but it is by no means guaranteed. This change allows us to use raw (i.e. `/websocket/`-free) request URIs if they're given from the server.

(This came up when debugging an issue on a server that wasn't using websocket URLs, and isn't urgent.)