mixed content with different charsets should be all converted to UTF-8

[jkbzh]

There are different scenarios when this may happen:

A message that has a multipart/mixed or message/rfc822 content with a given charset that differs from the charset of the enveloping message itself.

A header that has text encoded (RFC1342) in a non ASCII charset (RFC1342) that is different from that of the message.

The solution here is to convert everything to UTF-8, when hypermail is able to handle it.