Investigate precondition failed errors

[halkyon]

Sometimes we're seeing errors "At least one of the pre-conditions you specified did not hold" errors, which appear to come from minio around If-Modified headers, seemingly occuring when running a proxy in front of Gateway-ST and using the --website flag.

From a report elsewhere:

This ended up being caused by an nginx config change on our side, proxy_cache_revalidate. When set to on, nginx will send both If-Modified-Since and If-None-Match headers along with the validation request. For whatever reason, Gateway ST throws the previously shared error when it receives If-Modified-Since header.

[mitexleo]

What's causing this problem? I'm seeing the error msg