Fix path normalization of index route

Previously, `normalize_path` on `/` would result in an empty string, causing
routers to return 404 on the index route. Fix this by doing nothing if the
resultant string would be empty.

Fixes tower-rs#318.

tower-http/src/normalize_path.rs

@@ -114,6 +114,10 @@ fn remove_trailing_slash(uri: &mut Uri) {
 
     let new_path = uri.path().trim_end_matches('/');
 
+    if new_path.is_empty() {
+        return;
+    }
+
     let mut parts = uri.clone().into_parts();
 
     let new_path_and_query = if let Some(path_and_query) = &parts.path_and_query {
@@ -164,6 +168,13 @@ mod tests {
         assert_eq!(body, "/foo");
     }
 
+    #[test]
+    fn is_noop_on_index() {
+        let mut uri = "/".parse::<Uri>().unwrap();
+        remove_trailing_slash(&mut uri);
+        assert_eq!(uri, "/");
+    }
+
     #[test]
     fn is_noop_if_no_trailing_slash() {
         let mut uri = "/foo".parse::<Uri>().unwrap();