Make BaseModel.to_dict() return JSON-serializable values (#93)

[CedricConday]

Fixes #93

BaseModel.to_dict() is documented as returning the model properties as a dict, but enum-typed attributes come back as the raw Enum object, so:

json.dumps(journal.to_dict())
# TypeError: Object of type AccountType is not JSON serializable

serialize_to_dict (the singledispatch used by to_dict) has handlers for BaseModel, list, tuple and dict, but none for Enum, so enums fall through the identity default. The same applies to UUID fields (see #161).

Change

Register Enum and UUID handlers on serialize_to_dict (the Enum handler is exactly what was suggested in the issue):

@serialize_to_dict.register(Enum)
def serialize_enum_to_dict(value):
    return value.value


@serialize_to_dict.register(UUID)
def serialize_uuid_to_dict(value):
    return str(value)

Test

Added tests/test_models.py covering enum→value, UUID→string, and that json.dumps(model.to_dict()) no longer raises. The tests fail on master and pass with the fix; existing suite stays green.

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