Add unit tests for authentication and webhook parsing

- Implement tests for basic and bearer authentication headers in `test_auth.py`.
- Create tests for the `EasybillWebhookParser` in `test_webhooks.py`, covering JSON and form-encoded payloads, as well as a generic parse and acknowledgement method.

generated/sync/docs/ServiceDate.md

@@ -0,0 +1,34 @@
+# ServiceDate
+
+This object is only available in document type INVOICE or CREDIT.
+
+## Properties
+
+Name | Type | Description | Notes
+------------ | ------------- | ------------- | -------------
+**type** | **str** | With DEFAULT no other fields are required and this message will be printed: 'Invoice date coincides with the time of supply'.<br/> For SERVICE or DELIVERY exactly one of the following fields must be set: date, date_from and date_to or text. | [optional] 
+**var_date** | **date** |  | [optional] 
+**date_from** | **date** |  | [optional] 
+**date_to** | **date** |  | [optional] 
+**text** | **str** |  | [optional] [default to 'null']
+
+## Example
+
+```python
+from easybill_generated_sync.models.service_date import ServiceDate
+
+# TODO update the JSON string below
+json = "{}"
+# create an instance of ServiceDate from a JSON string
+service_date_instance = ServiceDate.from_json(json)
+# print the JSON string representation of the object
+print(ServiceDate.to_json())
+
+# convert the object into a dict
+service_date_dict = service_date_instance.to_dict()
+# create an instance of ServiceDate from a dict
+service_date_from_dict = ServiceDate.from_dict(service_date_dict)
+```
+[[Back to Model list]](../README.md#documentation-for-models) [[Back to API list]](../README.md#documentation-for-api-endpoints) [[Back to README]](../README.md)
+
+