Fix cache_peer login=PASS(THRU) after CVE-2015-5400

Fix cache_peer login=PASS(THRU) after CVE-2015-5400

The patch for CVE-2015-5400 converts all non-200 peer responses
into 502 Bad Gateway responses when relaying a CONNECT to a peer.

This happens to break login=PASS and login=PASSTHRU behaviour
which relies on the 401 and 407 status being relayed transparently.

We need to relay the auth server responses as-is when login= is
set to PASS or PASSTHRU but then unconditionally close the
connections to prevent CVE-2015-5400 from occuring.