smtplib.SMTP expects ASCII-only message bodies when message body is provided
as a 'str'. If we pass a 'bytes', we need to choose encoding earlier and we
do this by calling 'as_bytes' on messages with SMTP policy, which takes care
of formatting the body properly.
As a result, ISO-8859-x messages are converted to Quoted Printable and UTF-8
messages are Base64-encoded.
Testing this behaviour is tricky, because we use the same SMTP client to send
test data. For this reason, test code has become a bit ugly, but it does
exactly what we need.
Piotr F. Mieszkowski
b6bd36a460