Add some debug assertions (#686)

* Add some debug assertions

As discussed in Discord its probably worth having debug assertions for
these two cases, should we accidentally break them in the future.

* remove assertion in `OriginalUri`

axum/src/routing/mod.rs

@@ -417,6 +417,9 @@ where
                 Arc::clone(matched_path)
             };
             req.extensions_mut().insert(MatchedPath(matched_path));
+        } else {
+            #[cfg(debug_assertions)]
+            panic!("should always have a matched path for a route id");
         }
 
         let params = match_