HEAD plain CBOR codec with no explicit format returns HTTP 200

Implementation	bifrost-gateway	boxo	kubo
Conformance Version	main	main	main
Total	✅3	✅3	✅3
Header Content-Length	✅1	✅1	✅1
Header Content-Type	✅1	✅1	✅1
Status code	✅1	✅1	✅1