Supports nullable unique constraints

[tax]

Hi @michiya,

I added support for nullable unique constraints based on: http://stackoverflow.com/a/767702 .

Can you please review and let me know what you think.

Regards Paul

[rjschave]

@michiya,

I encountered this error last week w/ Django 1.9 and opened a ticket with Django (https://code.djangoproject.com/ticket/27627) before realizing this is an issue with this backend. I created a workaround using a custom migration. Obviously, adding support for nullable unique constraints would be the preferred course of action. Is there anything I can do to help get this pull request released (e.g. testing)?

Thanks.

[NBajanca]

Hi, is there any development here? I would like this feature as well.