Prior to this change, the `"Last-Modified"` and "`Etag`" support had
been improved with SPR-11324: HTTP response headers are now
automatically added for conditional requests and more.
This commit fixes the format of the "`Last-Modified`" and "`ETag`"
values, which were using an epoch timestamp rather than an HTTP-date
format defined in RFC 7231 section 7.1.1.1.
Also, Conditional responses are only applied when the given response
applies, i.e. when it has an compatible HTTP status (2xx).
Issue: SPR-13090