Define CoAP Content Negotiation

[ektrah]

Since the usage of contentType and contentCoding is really unclear and we don't seem to be making progress on this topic, I propose we just ditch that and define CoAP-specific vocabulary for content negotiation.

Closes #159

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[JKRhb]

You are right that having specific vocabulary for the content format and accept options probably makes more sense than deriving them from the given combination of contentType and contentCoding. Setting contentType to a zero length string does not seem that elegant to me, though.

Since contentType does not need to be present in a form but has a default value of application/json, could we rather say that CoAP forms should either not assume the default at all or a Content-Format of 50 instead which could be overridden by cov:available and cov:acceptable?

[relu91]

I don't like this solution either, I prefer to dig more into a proper solution rather than doing this kind of workaround. But it all depends on the priorities and the timeline of the WG. We had a long discussion in the last main call and it is unclear if we can continue to improve the Protocol Bindings Template documents in this charter. If everything is delayed to the new charter than I think we can find a consistent solution.

[ektrah]

(This PR relates to the discussion in w3c/wot-thing-description#1564.)

[ektrah]

Superseded by #193