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

[hedgehog125]

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?

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