This PR was squashed before being merged into the 5.4 branch. Discussion ---------- [HttpKernel] Correctly merge `max-age`/`s-maxage` and `Expires` headers | Q | A | ------------- | --- | Branch? | 5.4 | Bug fix? | yes | New feature? | no | Deprecations? | no | Issues | Fix contao/contao#7494 | License | MIT The `ResponseCacheStrategy` does not currently merge `Expires` and `Cache-Control: max-age/s-maxage` headers. Before #41665 this was not an issue, because if not all respones had all headers, they were not added to the final reponse. And we could assume a response itself is consistent between `Expires` and `max-age`. `@mpdude` added _heuristic caching_ of public responses in #41665. Unfortunately, it only looks at `Cache-Control: public` but if should also check if no cache information (max-age/s-maxage/Expires) is present. If that were the case, the behavior would not have changed. But it now leads to inconsistent header values because it independently keeps `Expires` and `max-age/s-maxage`. This PR does not only fix the _heuristic caching_, but also merges `Expires` and `Cache-Control` headers to make sure only the lowest value is retained across all headers. For semi-BC reasons I also made sure to only add an `Expires` header if any of the responses contains one. Commits ------- d3e65d6 [HttpKernel] Correctly merge `max-age`/`s-maxage` and `Expires` headers

Uh oh!