unit test of failing seralizers round tripping
value_to_string returns json string

not sure if anything is relying on the existing behavior of
value_to_string, at the very least the unit tests pass

- allows JSONField's to work correctly with django.core.serializers
- thus fixing fixtures, dumpdata/loaddata
Merge pull request #27 from ross/django_serializers

JSONField doesn't support django.core.seralizers, so fixtures don't work