move transformation of HttpServletRequest -> request-map into middleware

This allows people to write middlewares, like the one for implementing Servlet 3.0 Asynchronous responses below, that use the HttpServletRequest directly.

This functionality is used by https://github.com/ninjudd/ring-async to provide streaming responses without consuming a thread.

@weavejester, what are your thoughts? Another approach would be to just leave the `:servlet-request` in the request-map, but it seems that's how ring used to work, and a decision was made to move away from that.