Uses MvcUtils.setRouteId() rather than set attr by hand.

See gh-2949

spring-cloud-gateway-server-mvc/src/main/java/org/springframework/cloud/gateway/server/mvc/filter/BeforeFilterFunctions.java

@@ -314,7 +314,7 @@ public abstract class BeforeFilterFunctions {
 
 	public static Function<ServerRequest, ServerRequest> routeId(String routeId) {
 		return request -> {
-			request.attributes().put(MvcUtils.GATEWAY_ROUTE_ID_ATTR, routeId);
+			MvcUtils.setRouteId(request, routeId);
 			return request;
 		};
 	}