Strip all headers except X-Forwarded-For

[subDesTagesMitExtraKaese]

This ensures that nothing but the client IP can affect the response. Previously some request headers caused 304 and 412 responses.

Fixes #75 and passes their test cases:

200 OK (plain request)                        → 200
200 OK (If-Modified-Since)                    → 200
304 Not Modified (If-None-Match: *)           → 200
412 Precondition Failed (If-Match)            → 200
412 Precondition Failed (If-Unmodified-Since) → 200
403 X-Forwarded-For France is Blocked         → 403