Messages marked as "US-ASCII" and then exported to QWKnet, end up with CP437 chars in tagline

QWKnet taglines contain the "small box" character (CP437 254) and when these characters are added to an exported "US-ASCII" message, the included FTN CHRS header field isn't updated to reflect this, so the advertised character set does not match the actual character set of the complete message text.

Reported by Accession via IRC.