Fixed #3774 - primary_key=True does not, in fact, imply blank=False

Thus, you don't need to set ``primary_key=True`` on any of your fields

unless you want to override the default primary-key behavior.

``primary_key=True`` implies ``blank=False``, ``null=False`` and

``unique=True``. Only one primary key is allowed on an object.

``primary_key=True`` implies ``null=False`` and ``unique=True``. Only

one primary key is allowed on an object.

``radio_admin``

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