Archiver does not support GBK encoding

For Chinese mails encoded in GBK, many E-mail softwares errorously puts "GB2312" in the charset field of message header. GBK is a superset of GB2312, so decoding GBK message body with GB2312 charset will produce some wrong characters. There is no problem in decoding GB2312 messages with GBK charset.

The patch in attachment is quick-and-dirty: It simply regards every message which claims itself in GB2312 encoding as GBK encoded. This approach has been tested in 3 Chinese mailing lists.