Already rendered Email is re-rendered without context

[productaize]

In closing #403 a change was made to save the template independent of the render_on_delivery flag. This now causes the email to be re-rendered, however without the context. The result is the empty template message (placeholders are replaced with blank).

Either this should be changed to only re-render if Email.message or Email.html_message is empty, or the context should always be stored, as hinted here on 24 Aug

django-post_office/post_office/models.py

[zagl]

If the context is also just stored and nothing else changed it would be render_on_delivery=True by default.

If the context also "can be usefull for admin purposes" it should be saved and Email.message and Email.html_message should be checked for beeing empty.

If the context should not be saved the line

django-post_office/post_office/models.py

Line 108 in dddde78

if self.template is not None:

could simply be replaced by
if self.context is not None:

I created a pull request for the latter.

[gabn88]

This commit creates a problem with emails that have a context, but no template. I get "AttributeError: 'NoneType' object has no attribute 'subject'".

subject = engine.from_string(self.template.subject).render(self.context)