Add Request::url_mut method

[rvolosatovs]

Add url_mut method counterpart similar to how it's done for other components, e.g. https://docs.rs/tide/latest/tide/struct.Request.html#method.session_mut
Made sure the documentation is consistent with upstream library used https://docs.rs/http-types/latest/http_types/struct.Request.html#method.url

This is useful, for example, in cases where prefix of a route can contain forward-slashes and a request is to be routed to a nested server and custom routing relying on response method needs to be implemented.

For example, if /a/b/c/_myroute and /a/_myroute should call the same route on some Server, then an outer Server implementation could add a route on *, parse the URI from the request, trim the prefix as required and pass the request with modified URI (via url_mut) to the nested Server via Server::respond