Problem with default value when terms comes from a source

[tdesvenain]

Default value is excluded because terms is empty when value is validated

[tdesvenain]

An example that reproduces the issue:

@provider(IContextAwareDefaultFactory)
def test_default(context):
    return context.UID()

class IBroadcastForm(form.Schema):
    test = schema.Choice(
        title=u"test",
        defaultFactory=test_default,
        source=UUIDSourceBinder(),
    )
    form.widget(test=RadioFieldWidget)