Caching: Update existing Cache headers when a 304 is received.

[jasim]

Currently a 304 from server does not update the corresponding headers in the Cache entry. So after this resource has expired, every further request will need a server side validation.

But RFC 2616 says:
If a cache uses a received 304 response to update a cache entry, the cache MUST update the entry to reflect any new field values given in the response.

[jasim]

http://www.w3.org/Protocols/rfc2616/rfc2616-sec13.html#sec13.5.1

Only End-To-End headers must be updated.

[jasim]

Fixed in aa345d6