BooleanField.formfield() does not handle choices correct. [PATCH]
If you have a BooleanField, with choices (as in the example below), the formfield() method always returns a TypedChoicesField, with a blank choice at the beginning. This is because of this entry is included always if the blank is True and BooleanFields are always blank=True. So we have to check for null instead of blank, when using BooleanFields, since the blank flag in the BooleanfField does not specify if the field can be empty in forms in contrast to other fields.
FLAG_CHOICES = (
flag = models.BooleanField(choices=FLAG_CHOICES)