Fix redirect for mounted folders (#2628)

A directory request without a trailing slash was redirected to the path
relative to the mount point, so /mount/dir under a mount at /mount sent
Location: //dir/ instead of /mount/dir/. Redirect to the request path
with the slash appended.

httplib.h

@@ -13869,7 +13869,7 @@ inline bool Server::handle_file_request(Request &req, Response &res) {
         detail::FileStat stat(path);
 
         if (stat.is_dir()) {
-          res.set_redirect(sub_path + "/", StatusCode::MovedPermanently_301);
+          res.set_redirect(req.path + "/", StatusCode::MovedPermanently_301);
           return true;
         }