Fixed a bug causing `KeyError` when saving with the parameter `update_fields` in which there are untracked fields.
Merge remote-tracking branch 'silonov/save_with_untracked_updated_fields'

* silonov/save_with_untracked_updated_fields:
  Fixed a bug causing `KeyError` when saving with the parameter `update_fields` in which there are untracked fields.
Update testing idioms.