Update conformance tests to validate Version 1.0.1

[kevmoo]

https://github.com/cloudevents/spec/blob/v1.0.1/http-protocol-binding.md

At a minimum, sending Content-Type: application/cloudevents+json; charset=UTF-8

CC @grant

[grant]

We shouldn't need to update the tests against the latest CE version (just major version).

I don't know if the content type if application/json or application/cloudevent from Eventarc. I'd imagine we'd probably want to allow either content type, or not disallow it.