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.
This avoids problems with ESPs that don't distinguish *Content-ID*
from attachment filename, where a local hostname ending in ".com" could
cause Gmail to block messages sent with inline attachments.
(Mailgun, Mailjet, Mandrill and SparkPost have APIs affected by this.)
Fixes#112.
