Changes to Get Billing Contract

the response property end_date became optional for the status 200

the response property number_of_months became optional for the status 200

the response property start_date became optional for the status 200

response property end_date list-of-types was widened by adding types null to media type application/json of response 200

response property number_of_months list-of-types was widened by adding types null to media type application/json of response 200

response property start_date list-of-types was widened by adding types null to media type application/json of response 200

added the optional property billing_start_pending to the response with the 200 status