Support for Vary headers

[mihxil]

I think we should be able to define Vary-headers too.
E.g. the result to be cache may be dependent on accept-header or security information (which may be on headers).

[Shusshu]

@foo4u could you merge this one? It's very useful.

[mihxil]

The project seems to have been disbanded. I was needing also upgrading to spring 6 in a project using this. I just released my fork now: https://github.com/mihxil/spring-mvc-cache-control/. It also contains the fix for this issue.