update: ensure that the dumps method generates detached content COSE message

Author: kentakayama

tests/test_cose_message.py

@@ -502,6 +502,7 @@ def test_cose_message_detach_and_attach(self):
 
         detached_content_cose_message, detached_payload = ecdsa_cose_sign1_example.detach_payload()
         assert expected_detached_cose_message == detached_content_cose_message
+        assert expected_detached_cose_message.dumps() == detached_content_cose_message.dumps()
         assert expected_payload == detached_payload
 
         reverted_cose_message = detached_content_cose_message.attach_payload(detached_payload)