Fix 24h timeformat for `expire` and `last-modified` header

This patch was authored by @patrick-zippenfenig and released by @0xTim.

Expire and Last-Modified header were encoding the hour part in 12h format (hh) instead of 24h format (HH). This results in timestamps being 12 hours off for all afternoon hours. This fixes the format used to follow the spec correctly and adds tests to ensure no regressions.