Serialize UUID values instead of raising ValueError (#161)

[CedricConday]

Fixes #161

Calling e.g. accounting_api.get_invoice(...) can raise:

ValueError: Can't serialize value type 'UUID'

The singledispatch serializer in serializer.py has handlers for BaseModel and Enum but none for uuid.UUID, so any value that is a UUID (such as an invoice id) falls through to the default and raises.

Change

Register a UUID handler on serialize_model:

@serialize_model.register(UUID)
def serialize_uuid_model(model):
    return str(model)

I used str(model) (canonical 8-4-4-4-12 hyphenated form, e.g. 12345678-1234-5678-1234-567812345678) rather than the .hex form suggested in #93, because that is the GUID format the Xero API uses and accepts.

Test

Added test_serialize_uuid in tests/test_api_client/test_serializer.py. It fails on master (ValueError) and passes with the fix. Full test_api_client suite (151 tests) stays green.

Note: the related to_dict() path raised the same way for enums/UUIDs — fixed separately in #227.

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