Fixed: sending non-ascii message in Py3k

`smtplib.SMTP.sendmail` only accept `str` in ASCII range, or `bytes`.
This fix doesn't test if all characters in `msg_str` is in ASCII range,
just a simple force conversion to `bytes`

ref:
http://docs.python.org/3.3/library/smtplib.html#smtplib.SMTP.sendmail
https://code.djangoproject.com/ticket/19186