Clarify NATS binding content mode detection

[RAnders00]

Fixes #1398

Proposed Changes

• Changes the content-mode detection specified in the NATS binding to explicitly allow a structured syntax suffix as per RFC 6838.
• Sharpened the requirements around what "prefixed with" means. In the current wording, if the standard says "prefixed with application/cloudevents, one could understand that to include application/cloudevents-batch, which would clearly be incorrect.
• Also made the wording around case insensitivity more explicit, including a link to the related RFC and using the same wording as in the RFC.

Release Note

No release note is needed, since this detection was not part of v1.0.2.

[duglin]

ping @ColinSullivan1

[duglin]

@RAnders00 thanks for this. Am I correct that if we were to focus on "just the real concern", it's the lack of the word "prefixed" in the NATS section? The rest of it looks like editorial tweaks. Is that correct?

I'm not against editorial tweaks, but I want to make sure we have a clear distinction of "must" vs "nice to have" fixes since the group is trying to minimize changes to the CE specs.

[RAnders00]

@duglin Yeah, I think the NATS section needs amending. The other changes are, as you said, editorial tweaks because I looked more into this area and figured these things could be clarified. Apologies if that's uncalled for, I can undo those changes.

[duglin]

No the changes are fine - I just wanted to make sure I wasn't missing something.