Trailing slashes redirect doesn't check existence of folder

[michielbdejong]

The current implementation of #4 doesn't check whether the folder exists.
In hindsight I'm not so happy with that choice. I would like to give the PivotResponseWriter a way to check whether the folder exists in the storage, and only do the redirect if it does.
This will expose some information, but I think it's still a better option than serving redirects when the 401/403/404 response cannot be resolved by following those redirects.
A stretch goal would be to also check access; maybe that can be done with a rewrite of the DPoP Verifier, but we may have to leave that as room for future improvement.