fix: panic if add_route gets a path with a trailing slash

This will get surfaced by tests

src/router_ext.rs

@@ -1,6 +1,10 @@
 /// Naive macro to add a route and its trailing slash variant to the router
 macro_rules! add_route {
     ($router:ident, $route:literal) => {
+        assert!(
+            !$route.ends_with("/"),
+            "URLs passed to add_route must not end in /"
+        );
         $router.insert($route, ()).unwrap();
         $router.insert(concat!($route, "/"), ()).unwrap();
     };