full support of no-cache

[dbu]

since #26, the no-cache directive is correctly handled. the response is not cached. but no-cache is meant to force re-validation - the response might still be stored but needs to be revalidated each time.

AFAIK you can actually cache the result if response has no-cache directive. It just means you must always revalidate before actually using the cached result. If origin server returns 304 Not Modified you can use the cached result.

https://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.9.1