Circular dependency error in event info

[ggarnier]

For some events, the event info page raises a circular dependency error. This test reproduces the error:

from tsuru_dashboard.apps.views import event_serialization_default

class EventInfoViewTest(TestCase):
    def test_circular_dependency(self):
        data = {'_id': bson.ObjectId('5b64c49ad32e70370e94422a')}
        # This raises ValueError: Circular reference detected
        json.dumps(data, default=event_serialization_default)