Support for named groups in choices

[jamesandres]

This PR fixes an issue where named groups in choices would cause validation errors.

For example, this now works:

PROVINCES = (
    ("AB", "Alberta"),
    ("BC", "British Columbia"),
    # ... snip ...
)
STATES = (
    ("AL", "Alabama"),
    ("AK", "Alaska"),
    ("AZ", "Arizona"),
    # ... snip ...
)
PROVINCES_AND_STATES = (
    ("Canada - Provinces", PROVINCES),
    ("USA - States", STATES),
)

class Address(models.Model):
    # ... snip ...
    province_state = MultiSelectField(
        _('Province or State'), max_length=2, choices=PROVINCES_AND_STATES)
    # ... snip ...

[coveralls]

Coverage decreased (-2.12%) when pulling b7671d6 on jamesandres:named-groups into c232d17 on goinnn:master.

[blag]

Add a test for this and I'll merge it in. Thanks!

[blag]

I cherry-picked this into master and added a test for it. Thanks for your contribution! 😄

[jamesandres]

Thanks @blag! Sorry I haven't been so helpful, I wrote this PR to scratch an itch while working on a project quite a while back.

[blag]

@jamesandres No worries, merging it was easy! If you get a chance, test out the latest release and let me know how it is. 😄