NullBooleanField should be treated as BooleanField(null=True, blank=True)

[satyanash]

Currently when using _fill_optional=True, Model Mommy will randomly choose between (None, False, True) for a NullBooleanField.
This feature was added in #305, but I think if _fill_optional=True is specified, it should treat a NullBooleanField the same as a BooleanField(null=True, blank=True) as Django recommends using NullBooleanField where there is a use case for the empty state in a boolean field.

Model Mommy treats None here as a valid non-empty value even though it still means that the field is empty. This confuses any tests that assume that _fill_optional=True will guarantee no None values.

[Nuurek]

Somehow related: https://docs.djangoproject.com/en/dev/releases/2.1/

BooleanField can now be null=True. This is encouraged instead of NullBooleanField, which will likely be deprecated in the future.

[berinhard]

I'm a 👍 for this change in model mommy.