Add: failing test for html only messages
Fix: html messages were sent as text

If you followed the instructions at:
<https://docs.djangoproject.com/en/1.5/topics/email/#sending-alternative-content-types>,
djrill would send the body as 'text' instead of 'html', resulting in
ugly emails.

This also fixes how `[auto_text]` could be set, but couldn't do anything.
If you wanted to use `auto_text=True`, you would have to send a null
body and then `attach_alternative` your real body. I didn't add a test
case for this because they're like... orthogonal..., and gets into
behavior testing vs unit testing.

  [auto_text]: https://djrill.readthedocs.org/en/master/usage/sending_mail/#auto_text
Merge pull request #45 from texastribune/support-html-alternate-content

Fix: Djrill assumes message content type is text