Conditional requests using "If-Unmodified-Since" headers are generally
used as precondition checks for state-changing methods (POST, PUT,
DELETE). See https://datatracker.ietf.org/doc/html/rfc7232#section-3.4
The spec also allows for idempotent methods like GET and HEAD.
Prior to this commit, the "If-Unmodified-Since" processing done in
`checkNotModified` (see `ServletWebRequest` and
`DefaultServerWebExchange`) would only focus on the state changing
methods and not take into account the safe methods. For those cases, the
"ETag" and "Last-Modified" would be missing from the response.
This commit ensures that such headers are added as expected in these
cases.
Fixes gh-29362
9410998897