always set defaults in behavior config
always set defaults in behavior config

When a behavior adds a column but doesn't set a default in their inner
config class, we should add it for them to improve the access code.

    class Foo(Behavior):
        column = models.IntegerField()

ends up as if they had written

    class Foo(Behavior):
        class Foo:
            column = 'column'
        column = models.IntegerField()

this allows the behavior author to write Foo.Foo.column without writing
code to check if it's set or not.

fusionbox/django-argonauts