bug#45925: 27.1; *Summary* buffer vs. raw utf-8 headers

[積丹尼 Dan Jacobson]

Try this simple experiment:
$ echo Subject: 一二三|procmail
$ echo Subject: 一二三|iconv -t big5|procmail
$ emacs -f gnus

In the *Article* buffer, both look like
Subject: 一二三
In the *Summary* buffer so does the big5 version.
Alas, the utf-8 version looks like
c\x80\xd3....

(Yes, these are illegal raw headers. But Gnus is supposed to be
accommodating. And it does... but oddly not for the majority (UTF-8) case.)

Important settings:
  value of $LC_COLLATE: C
  value of $LC_CTYPE: zh_TW.UTF-8
  value of $LC_MESSAGES: C
  value of $LANG: zh_TW.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

(Might be related to bug#45724.)

(https://www.jidanni.org/comp/configuration/ has my dot files. )