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

djrill/mail/backends/djrill.py

@@ -112,8 +112,9 @@ class DjrillBackend(BaseEmailBackend):
         to_list = [{"email": to_email, "name": to_name}
                    for (to_name, to_email) in parsed_rcpts]
 
+        content = "html" if message.content_subtype == "html" else "text"
         msg_dict = {
-            "text": message.body,
+            content: message.body,
             "subject": message.subject,
             "from_email": from_email,
             "to": to_list