delivery on read reciepts violate rfc2045

SendMDN function in src/read_body.php sets encoding to
us-ascii. Japanese translation uses extra functions
that set 7bit or 8bit.

Emails are correctly formated only in English and
Japanese translations. Other translations use 8bit
strings and system sets Content-Transfer-Encoding=us-ascii.

Same thing for 1.4.3cvs and 1.5.1cvs. (2004-04-12)