update OPTIONS responses to conform to API-X proposal
See, for instance, this document: https://docs.google.com/document/d/1PMM7Q6WVF6QrsyQ2YKoJItfj4w8DDPPx8rpFJVxEXco/edit?ts=5758bcd0
See, for instance, this document: https://docs.google.com/document/d/1PMM7Q6WVF6QrsyQ2YKoJItfj4w8DDPPx8rpFJVxEXco/edit?ts=5758bcd0