Added support content_subtype

https://docs.djangoproject.com/en/1.9/topics/email/#sending-alternative-content-types

Author: haos616

sparkpost/django/message.py

@@ -25,10 +25,14 @@ def __init__(self, message):
         formatted = {
             'recipients': message.to,
             'from_email': message.from_email,
-            'subject': message.subject,
-            'text': message.body
+            'subject': message.subject
         }
 
+        if message.content_subtype == 'html':
+            formatted['html'] = message.body
+        else:
+            formatted['text'] = message.body
+
         if message.cc:
             formatted['cc'] = message.cc