下面这个邮件头是新闻组中收来的，
帖子的内容已经转换成 gbk 编码了，
但是charset为什么还是big5?
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Path: news.happynet.org!news.newsmth.net!ptt
Message-ID: <4R6jPh$hSN@ptt.cc>
From: Blue6.bbs@ptt.cc (不入流)
Newsgroups: cn.bbs.lang.chinese
Subject: Re: 古代女性的自称？
Date: 12 Nov 2006 06:07:18 +0800 (CST)
Organization: 批踢踢实业
Mime-Version: 1.0
Content-Type: text/plain; charset=big5
Content-Transfer-Encoding: 8bit
X-Filename: Chinese/M.1163311579.A.82C
NNTP-Posting-Host: 59.115.83.86
Xref: emacsfans.org cn.bbs.lang.chinese:103
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FROM 59.66.78.*