Inconsistent API of django's EmailBackend._send

[sjoerdjob-legalsense]

Hi! In #89 , _send was changed to return a tuple, and send_messages was updated to expect a tuple.

However: looking at https://github.com/themartorana/python-postmark/pull/89/files#diff-0a79263ddca0f892c07ae4bdfe5201366d5e7dd9b0fc7d23be8937a1b9dce1edR142 we see that there are still cases where the _send method returns a single boolean instead of a tuple of a boolean together with a list of messages. This then causes a TypeError in send_messages.

[nicholasserra]

Good catch. I'll get a patch and new version up asap. Sounds like we're missing some test coverage.

[nicholasserra]

Fix in #106