[imp] The preferred charset feature is incomplete

[Jinhyok Heo]

I am happy to find that IMP supports a preferred character set for
reading emails since 4.1.1. But it is incomplete.

There is no problem in reading a mail(message.php), but characters are
still broken in a list page(mailbox.php).

I've tested with the latest, IMP 4.1.2 on Horde 3.1.2.

-- 
Jinhyok Heo