The logger can throw UnicodeEncodeError if the log message is unicode

[Mark Sapiro]

There is code in Mailman's _Syslog.write_ex() method to catch a UnicodeError when writing the log file and quoted-printable encode the log message. This works on messages that contain non-ascii, 8-bit bytes, but if the message is a unicode with non-ascii characters, the attempt to quoted-printable encode it will throw UnicodeEncodeError which is not caught.

Fix this by first encoding the unicode message as utf-8 and then quoted-printable encoding that.