Make websocket path relative to Shiny page path

This is necessary for proxy situations that don't override the
Shiny.createSocket function (so, not including Shiny Server,
but more like Nginx, HAProxy, and a future version of RStudio
Server).

Without the path being preserved, it's impossible for these
proxies to know that the URL should be forwarded to the host
and port that belongs to Shiny.