bug#49115: Mumi inserts spurious underscore in bug title

[Ricardo Wurmus <rekado@elephly.net>]

> See http://issues.guix.gnu.org/49114, which Mumi currently 
> titles
>
>   ‘guix_ lint’ should catch certificate validation exceptions
>
> There's no underscore in the original subject.

The debbugs “.log” file for issue 49114 contains this line:

  Subject: bug#49114: =?UTF-8?Q?=E2=80=98guix_?= =?UTF-8?Q?lint=E2=80=99?= should catch certificate validation exceptions

This is MIME Q encoding (similar to “quoted printable” encoding), as
specified in RFC 2047.

The underscore is mentioned in 4.2 (2):

   (2) The 8-bit hexadecimal value 20 (e.g., ISO-8859-1 SPACE) may be
       represented as "_" (underscore, ASCII 95.).  (This character may
       not pass through some internetwork mail gateways, but its use
       will greatly enhance readability of "Q" encoded data with mail
       readers that do not support this encoding.)  Note that the "_"
       always represents hexadecimal 20, even if the SPACE character
       occupies a different code position in the character set in use.

So, the underscore above ought to be replaced with a space.  This seems
to be a bug in guile-email.

-- 
Ricardo