If you are using an SES ConfigurationSet with open or click tracking
enabled, SES replaces non-ASCII characters with question marks as it
rewrites the message to add tracking, if the bodies are sent with
`Content-Transfer-Encoding: 8bit` (which is Django's default for utf8
body parts).
Force potentially problematic parts to use CTE: quoted-printable
as a workaround.
Fixes#115.