json_output_decorator fails on enums

[Aspect26]

When json_output_decorator receives a model, which contains enum values, it is unable to serialize it to json and crashes with Circular reference detected

Traceback (most recent call last):
  File "/usr/local/lib/python3.8/site-packages/zsl/application/modules/cli_module.py", line 44, in task
    result = exec_task(task_path, data)
  File "/usr/local/lib/python3.8/site-packages/zsl/interface/task.py", line 137, in exec_task
    return jc.task_callable(jc.task_data)
  File "/usr/local/lib/python3.8/site-packages/zsl/task/task_decorator.py", line 98, in json_input_decorator
    return f(*args, **kwargs)
  File "/usr/local/lib/python3.8/site-packages/zsl/task/task_decorator.py", line 122, in json_output_decorator
    rv = json.dumps(rv, cls=AppModelJSONEncoder, indent=indent)
  File "/usr/local/lib/python3.8/json/__init__.py", line 234, in dumps
    return cls(
  File "/usr/local/lib/python3.8/json/encoder.py", line 199, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/usr/local/lib/python3.8/json/encoder.py", line 257, in iterencode
    return _iterencode(o, 0)
ValueError: Circular reference detected