HTTP/1.1: Update all stored headers on 304 revalidation.

HTTP/1.1: Update all stored headers on 304 revalidation.

According to RFC 7234 section 4.3.4. when a proxy receives 304 (Not Modified)
response, it should update every header, except Warning headers (which need
special processing). Also RFC 7232 section 4.1. does not prohibit origin servers
from sending any headers in 304 responses. That means that Squid now should not
ignore headers, which earlier were considered as malicious/faulty.