fix the misplacement of adding slashes (#2847)

tree.go

@@ -634,7 +634,8 @@ walk: // Outer loop for walking the tree
 		// Nothing found. We can recommend to redirect to the same URL with an
 		// extra trailing slash if a leaf exists for that path
 		value.tsr = path == "/" ||
-			(len(prefix) == len(path)+1 && n.handlers != nil)
+			(len(prefix) == len(path)+1 && prefix[len(path)] == '/' &&
+				path == prefix[:len(prefix)-1] && n.handlers != nil)
 		return
 	}
 }