HTTP: do not allow Proxy-Connection to override Connection header

HTTP: do not allow Proxy-Connection to override Connection header

Proxy-Connection header is never actually valid, it is relevant in
HTTP/1.0 messages only when Connection header is missing and not
relevant at all in HTTP/1.1 messages.

This fixes part of the behaviour, making Squid use Connection header
for persistence (keep-alive vs close) checking if one is present
instead of letting Proxy-Connection override it.

TODO: Proxy-Connection still needs to be ignored completely when
      the message version is HTTP/1.1.