If user doesn't modify the replied-to-msg subject, use the original subject

(which might be UTF-8 encoded). This works-around the problem that Accession
reported in #synchronet with my reply to a UTF-8 encoded message using a CP437
terminal which resulted in a message body that was UTF-8 encoded but a message
subject that was CP437 encoded. This mix of encodings is not supported by FTN
standards.

This is just a work-around since if the user modifies the subject the result
could still have the CP437 unside-down question marks (indicating
non-translatable UNICODE chars) and those should be converted to UTF-8
chars when going out on FTN or being stored in the message base. So there's
still a bug here somewhere that I need to look into more.