Prior to this commit, Spring MVC and Spring WebFlux would not support
conditional requests with `If-Match` preconditions. As underlined in the
RFC9110 Section 13.1, those are related to the `If-None-Match`
conditions, but this time only performing requests if the resource
matches the given ETag.
This feature, and in general the `"*"` request Etag, are generally
useful to prevent "lost updates" when performing a POST/PUT request: we
want to ensure that we're updating a version with a known version or
create a new resource only if it doesn't exist already.
This commit adds `If-Match` conditional requests support and ensures
that both `If-Match` and `If-None-Match` work well with `"*"` request
ETags.
We can't rely on `checkNotModified(null)`, as the compiler can't decide
between method variants accepting an ETag `String` or a Last Modified
`long`. Instead, developers should use empty ETags `""` to signal that
no resource is known on the server side.
Closes gh-24881
0783f0762d