Fix handling of missing 'content-transfer-encoding' header

Closes: #136

Python's email module raises a KeyError if the header
'content-transfer-encoding' is missing:
  https://bugs.python.org/issue27321

django_mailbox/models.py

@@ -225,6 +225,8 @@ class Mailbox(models.Model):
     def process_incoming_message(self, message):
         """Process a message incoming to this mailbox."""
         msg = self._process_message(message)
+        if msg is None:
+            return None
         msg.outgoing = False
         msg.save()
 
@@ -235,6 +237,8 @@ class Mailbox(models.Model):
     def record_outgoing_message(self, message):
         """Record an outgoing message associated with this mailbox."""
         msg = self._process_message(message)
+        if msg is None:
+            return None
         msg.outgoing = True
         msg.save()
         return msg
@@ -363,7 +367,14 @@ class Mailbox(models.Model):
             )
         msg.save()
         message = self._get_dehydrated_message(message, msg)
-        msg.set_body(message.as_string())
+        try:
+            body = message.as_string()
+        except KeyError as exc:
+            # email.message.replace_header may raise 'KeyError' if the header
+            # 'content-transfer-encoding' is missing
+            logger.warning("Failed to parse message: %s", exc,)
+            return None
+        msg.set_body(body)
         if message['in-reply-to']:
             try:
                 msg.in_reply_to = Message.objects.filter(
@@ -382,6 +393,7 @@ class Mailbox(models.Model):
             return new_mail
         for message in connection.get_message(condition):
             msg = self.process_incoming_message(message)
+            if not msg is None:
                 new_mail.append(msg)
         self.last_polling = now()
         if django.VERSION >= (1, 5):  # Django 1.5 introduces update_fields