[Cache] Vary

[dbu]

the CachePlugin currently does not respect the Vary header. vary by its nature can not be part of the cache key (we don't know the vary from the request, only from the response). but we should check the response we find for a Vary header and compare the headers from the original request with the ones from the current request.

in many scenarios, we will never vary (e.g. different accept-encodings are unlikely) but e.g. when sending requests on behalf of a logged in user, we might do vary.