http caching validation is not RFC7232 compliant

[Smixi-syn]

Hi,

I figured out that the precedence rules in the HTTP RFC is not correclty implemented https://datatracker.ietf.org/doc/html/rfc7232#section-6

In my understanding, it should not check the If-Modified-Since condition whenever someone is also giving an Etag.

I made a PR for this if you want to understand it (#2891).

I saw this by using serving files with broken timestamps, the etag were not matching, but the Last modified were valids, giving me the wrong file version, tested out with "cache-control: no-cache" header set ot ensure the browser behavior. I did see the If-Modified-Since and the Etag headers sent out.