DjrillMessage class fixes

* Don't crash if no tags
* Allow `None` to omit options entirely from Mandrill send call
* Default `preserve_recipients` to None (= use setting from Mandrill account)
* ImproperlyConfigured --> ValueError for bad tags

djrill/mail/__init__.py

@@ -1,4 +1,3 @@
-from django.core.exceptions import ImproperlyConfigured
 from django.core.mail import EmailMultiAlternatives
 
 
@@ -8,23 +7,28 @@ class DjrillMessage(EmailMultiAlternatives):
     def __init__(self, subject='', body='', from_email=None, to=None, bcc=None,
         connection=None, attachments=None, headers=None, alternatives=None,
         cc=None, from_name=None, tags=None, track_opens=True,
-        track_clicks=True, preserve_recipients=False):
+        track_clicks=True, preserve_recipients=None):
 
         super(DjrillMessage, self).__init__(subject, body, from_email, to, bcc,
             connection, attachments, headers, alternatives, cc)
 
-        self.from_name = from_name
-        self.tags = self._set_mandrill_tags(tags)
-        self.track_opens = track_opens
-        self.track_clicks = track_clicks
-        self.preserve_recipients = preserve_recipients
+        if from_name:
+            self.from_name = from_name
+        if tags:
+            self.tags = self._set_mandrill_tags(tags)
+        if track_opens is not None:
+            self.track_opens = track_opens
+        if track_clicks is not None:
+            self.track_clicks = track_clicks
+        if preserve_recipients is not None:
+            self.preserve_recipients = preserve_recipients
 
     def _set_mandrill_tags(self, tags):
         """
         Check that all tags are below 50 chars and that they do not start
         with an underscore.
 
-        Raise ImproperlyConfigured if an underscore tag is passed in to
+        Raise ValueError if an underscore tag is passed in to
         alert the user. Any tag over 50 chars is left out of the list.
         """
         tag_list = []
@@ -33,8 +37,8 @@ class DjrillMessage(EmailMultiAlternatives):
             if len(tag) <= 50 and not tag.startswith("_"):
                 tag_list.append(tag)
             elif tag.startswith("_"):
-                raise ImproperlyConfigured(
+                raise ValueError(
                     "Tags starting with an underscore are reserved for "
-                    "internal use and will cause errors with Mandill's API")
+                    "internal use and will cause errors with Mandrill's API")
 
         return tag_list