Fix stat for bucket root

[lorenz]

Currently afero-s3 doesn't let clients list the bucket root because a HEAD on it doesn't return a 404 unless the bucket itself doesn't exist which means the existing code assumed it was a file. The bucket root is a directory by definition though, so this exposes it as such.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.97%. Comparing base (13198e5) to head (f83cb02).
Report is 408 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #393      +/-   ##
==========================================
- Coverage   81.44%   80.97%   -0.47%     
==========================================
  Files           3        3              
  Lines         345      347       +2     
==========================================
  Hits          281      281              
- Misses         40       41       +1     
- Partials       24       25       +1     

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[lorenz]

@fclairamb Could you have a look at this change? It's a three-line fix and I'm now getting private messages by people who also need it on how to integrate it.