_default_manager should be used instead of objects

[ChickenF622]

Ran into an issue where if I used a custom manager ClusterableModels will sometimes throw an error when using the custom manager. It appears the the get_queryset function in fields.py specifically line 337 is defaulting to using objects instead of _default_manager. The issue can be worked around by assigning an objects manager, but _default_manager should be used instead.