66930fd8 switched from an explicit _utf8_off to an explicit _utf8_on, in
an attempt to switch from splitting on bytes to splitting on characters.
However, the "UTF8" flag does not magically determine if a string is
bytes or characters.  Instead, only consistency in calling convention
can do so.  All callsites of RT::Interface::Email::EncodeToMIME and
RT::Action::SendEmail::MIMEEncodeString now pass character strings; all
that _utf8_on can do is incorrectly "decode" those strings as UTF-8 if
they happen to not have the "UTF8" flag set.