Add a new attribute 'listings_url' for the listing search URL that points to the old API and will be deprecated.
Use the 'assertIsInstance' method instead of checking for type.
For the test utils method 'wrap_content' allow the optional headers parameter.
Add missing required attribute 'title' to the Property Group response data.
Use the new utility method 'wrap_data_response' to correctly format the data being used to create each individual Resource in the 'get_items' method.
Add tests for the new 'listings_url' attribute.