django 1.8+ compat to ensure to_python is always called when accessing result from db..

[sbussetti]

Without this extra_data field can be returned as its raw serialized string...

[clintonb]

This issue was introduced by #813. Django docs for reference: https://docs.djangoproject.com/en/1.9/howto/custom-model-fields/#converting-values-to-python-objects.

[sbussetti]

@clintonb yeah, the author of that patch removed SubFieldBase without reading the rest of the deprecation instructions from the Django docs which instruct you to provide from_db_value going forward. Not that it matters to me, but #813 would also break backwards compat...