ISPN-3206 REST endpoint returns Expiry header in default locale for the server

This should be pulled after https://github.com/infinispan/infinispan/pull/1879 otherwise there will be conflicts.

* Due to a bug in RESTEasy, set the Expires and Last-Modified headers manually (besides that, Last-Modified's generation is correct in 2.3.2 version (which is currently used by ISPN) but in recent versions it uses toString on the passed date to generate the header -> if we ever upgrade to newer RESTEasy, we'll be safe)