Rewrote the tests to keep all the existing tests working against an unmodified SimSMTPServer, so that we know backward compatibility is maintained.  And as I said, I changed the tests to reflect that a non-ascii body must be provided as binary data, which means I deleted two tests.

(Note: in thinking about this, I vaguely remember some conversations where we thought it might make sense that if someone provided a message string that when encoded to utf-8 then conforms to the RFC, it would be OK to auto-encode it as utf-8, but I'm just not comfortable with that idea.  It provides insufficient value compared to the danger of blindly encoding something that is not RFC conformant...if someone does build a such a unicode message, they should have to convert it themselves before passing it to smtplib, as a sign they know what they are doing :)

Thanks for your work Milan.