#2830 UTF-8 encoding in text/plain mishandled

[Andy Valencia]

When I receive an email in text/plain with UTF-8 encoding, it mismatches my own (ASCII-ish) encoding.
The UTF-8 code point drops into my message body as the bytes of the code point. When I send back,
my message is text/plain with ASCII, thus the UTF-8 code point won't be interpreted by the recipient.

Ideally, Squirrelmail should down-convert the UTF-8 to ASCII, so things like right-single-quote
become a plain ASCII single quote.