Fixed another encoding issue.
Fixed another encoding issue.

This time we have to stick to ascii and thus need to use the backslashreplace errors handler.

Example of previously failing command (this would need a unit test):

    RCPT TO: François KUBLER <francois+github@kubler.org>

Now becomes the valid :

    b'RCPT TO: Fran\\xe7ois KUBLER <francois+github@kubler.org>'

Frzk/smtplibaio,hwmrocker/smtplibaio