Preserve option fields after setup

Current implementation of rack-parser would remove `:parsers`, `:handlers` and `:logger` fields after initialization.

While this might be okay for situations that uses `dup` to create multiple app instances, it might bring problems if we are re-initializing the app instance from scratch again(like in tests).

So, is there any particular reasons that `delete` is used here? If not, can we remove the use of `delete` here?