Spring Support date properties in Content-Disposition HTTP header

As discussed with @rstoyanchev, before merging this PR we should choose a strategy regarding to date/time that we previously exposed with long like in long HttpHeaders.getAccessControlMaxAge().

Should we continue to use long for consistency? Should we add Instant based variants and deprecate long based ones?

Pros and cons should be discussed and the solution should be applied globally to Spring Framework API like we did for Optional.

cc @jhoeller

Issue: SPR-15555

Comment From: sdeleuze

Merged via https://github.com/spring-projects/spring-framework/commit/97909f225819b2297c1036a8a9108c0582edcf90.