Smarter auth_param utf8 handling, including CP1251 support (#480)
When forwarding authentication credentials to authentication helpers:
* With auth_param utf8 parameter: Squid assumed that the received
credentials are encoded with Latin-1 (and re-encoded them with UTF-8).
This assumption is wrong for Internet Explorer running with CP1251
regional settings. Now Squid uses HTTP Accept-Language request header
to guess the received credentials encoding (Latin-1, CP1251, or UTF-8)
and converts the first two encodings into UTF-8.
* Without auth_param utf8 parameter: No changes. Squid sends credentials
in their original encoding, only applying RFC 1738 escaping on top.
Chrome and Firefox should not be affected because they always use UTF-8
encoding when sending authentication credentials.