Without this, `DebugLocks` middleware raises an error as follwing:

```
Puma caught this error: can't modify frozen String (RuntimeError)
actionpack/lib/action_dispatch/middleware/debug_locks.rb:97:in `block in render_details'
actionpack/lib/action_dispatch/middleware/debug_locks.rb:64:in `each'
actionpack/lib/action_dispatch/middleware/debug_locks.rb:64:in `map'
actionpack/lib/action_dispatch/middleware/debug_locks.rb:64:in `render_details'
actionpack/lib/action_dispatch/middleware/debug_locks.rb:37:in `call'
railties/lib/rails/engine.rb:524:in `call'
```