[bugfixed] mm-charset-override-alist isn't used in decoding headers

[Katsumi Yamaoka <yamaoka@jpl.org>]

Hi,

I receive Japanese mails that are labeled with charset=gb2312,
however most of them use charset=gbk in reality.  So, Gnus normally
fails in decoding those messages.  Adding the `(gb2312 . gbk)' pair
to `mm-charset-override-alist' solves it in decoding bodies, however
it is not effective in decoding headers.  Here is an example:

(rfc2047-decode-string "=?gb2312?b?srubcg==?=")

I've fixed the rfc2047 decoder so as to use `mm-charset-override-alist'.

In addition, I've added the `(gb2312 . gbk)' pair to the custom
choices (not the default value) of `mm-charset-override-alist'.
Although the `gbk' charset is a superset of the `gb2312' charset,
the reason I didn't add the pair to the default value of
`mm-charset-override-alist' is that the `gbk' is not available in
Emacs 21 normally and there seemed to be no smart way to exclude
it in such Emacs versions.

(Note that that I mention here is the `gbk' charset, not the `gbk'
 coding system.  So, `mm-charset-to-coding-system' is the best
 choice for checking whether it is available or not.  However,
 when loading mm-util,elc the default value section and/or the
 `:set' section of `(defcustom mm-charset-override-alist ...)'
 are processed before defining that function.)

Regards,