Fix path normalization for preceding slashes

fixes #358

tower-http/src/normalize_path.rs

@@ -108,21 +108,15 @@ where
 }
 
 fn normalize_trailing_slash(uri: &mut Uri) {
-    if !uri.path().ends_with('/') {
+    if !uri.path().ends_with('/') && !uri.path().starts_with("//") {
         return;
     }
 
-    let new_path = uri.path().trim_end_matches('/');
+    let new_path = format!("/{}", uri.path().trim_matches('/'));
 
     let mut parts = uri.clone().into_parts();
 
     let new_path_and_query = if let Some(path_and_query) = &parts.path_and_query {
-        let new_path = if new_path.is_empty() {
-            "/"
-        } else {
-            new_path.into()
-        };
-
         let new_path_and_query = if let Some(query) = path_and_query.query() {
             Cow::Owned(format!("{}?{}", new_path, query))
         } else {
@@ -218,4 +212,18 @@ mod tests {
         normalize_trailing_slash(&mut uri);
         assert_eq!(uri, "/?a=a");
     }
+
+    #[test]
+    fn removes_multiple_preceding_slashes_even_with_query() {
+        let mut uri = "///foo//?a=a".parse::<Uri>().unwrap();
+        normalize_trailing_slash(&mut uri);
+        assert_eq!(uri, "/foo?a=a");
+    }
+
+    #[test]
+    fn removes_multiple_preceding_slashes() {
+        let mut uri = "///foo".parse::<Uri>().unwrap();
+        normalize_trailing_slash(&mut uri);
+        assert_eq!(uri, "/foo");
+    }
 }