Fixes a HTML content problem uncovered from django-post-office 3

[clickonchris]

Allows for sending EmailMultiAlternatives messages with content_subtype html and without text/html alternative

tests mimic django-post-office 3 implementation here
https://github.com/ui/django-post_office/blob/v3.2.1/post_office/models.py#L126

[clickonchris]

this is the only substantial change. The rest is just django 2 wiring and tests

[nicholasserra]

Hey Chris, thanks for the PR! I'll check this out early next week. Looks good on quick peek.

To be clear, the issue was that you couldn't have a EmailMultiAlternatives message send without text/html?

[clickonchris]

you’re welcome. The issue my team experienced was- with Django post office if you have html content but no text content it was sending the html AS text so the recipient got a plain text email full of html markup. I.o.w EmailMultiAlternative with text/html content subtype but *without* a text/html alternative defined was being treated as a plain text email even if it contained html content

…

On Fri, Jan 17, 2020 at 5:31 PM Nicholas Serra ***@***.***> wrote: Hey Chris, thanks for the PR! I'll check this out early next week. Looks good on quick peek. To be clear, the issue was that you couldn't have a EmailMultiAlternatives message send without text/html? — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <#82?email_source=notifications&email_token=AAC273RBRVSMDVFHEUZJHGDQ6I5T7A5CNFSM4KHIKT7KYY3PNVWWK3TUL52HS4DFVREXG43VMVBW63LNMVXHJKTDN5WW2ZLOORPWSZGOEJJIRPQ#issuecomment-575834302>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAC273XO4B3OZM73RQUMFQ3Q6I5T7ANCNFSM4KHIKT7A> .