Clean paths before calling the http.FileSystem passed to router.StaticFS

[NicoClack]

Fixes the issues I describe in #4451

Some questions:

• Is this considered a breaking change? It changes some behaviour but mostly fixes it
• Are the subtleties of this correct? I adapted http.Dir but maybe we want to implement the slightly simpler fix in http.FileServer instead?
• Should the test use embedding? I'm copying gin-contrib/static a bit, but that makes it a bit more realistic?

Pull Request Checklist

Please ensure your pull request meets the following requirements:

• [ ✔️ ] Open your pull request against the master branch.
• [ ❌ ] All tests pass in available continuous integration systems (e.g., GitHub Actions).
• • I think there might be some issues with the CI?
• [ ✔️ ] Tests are added or modified as needed to cover code changes.
• [ N/A ] If the pull request introduces a new feature, the feature is documented in the docs/doc.md.

Thank you for contributing!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.99%. Comparing base (3dc1cd6) to head (c19d29e).
⚠️ Report is 230 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4452      +/-   ##
==========================================
- Coverage   99.21%   98.99%   -0.22%     
==========================================
  Files          42       44       +2     
  Lines        3182     2991     -191     
==========================================
- Hits         3157     2961     -196     
- Misses         17       21       +4     
- Partials        8        9       +1     

Flag	Coverage Δ
	?
--ldflags="-checklinkname=0" -tags sonic	98.98% <100.00%> (?)
-tags go_json	98.92% <100.00%> (?)
-tags nomsgpack	98.98% <100.00%> (?)
go-1.18	?
go-1.19	?
go-1.20	?
go-1.21	?
go-1.24	98.99% <100.00%> (?)
go-1.25	98.99% <100.00%> (?)
macos-latest	98.99% <100.00%> (-0.22%)	⬇️
ubuntu-latest	98.99% <100.00%> (-0.22%)	⬇️

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