Change Router::with_state and impl Service for Router<()> (#1552)

* Implement `Service` for `Router<(), B>`

* wip

* wip

* fix some tests

* fix examples

* fix doc tests

* clean up docs

* changelog

* fix

* also call `with_state` when converting `MethodRouter` into a `MakeService`

* suggestions from review

examples/rest-grpc-multiplex/src/main.rs

@@ -55,7 +55,7 @@ async fn main() {
         .init();
 
     // build the rest service
-    let rest = Router::new().route("/", get(web_root)).into_service();
+    let rest = Router::new().route("/", get(web_root));
 
     // build the grpc service
     let grpc = GreeterServer::new(GrpcServiceImpl::default());