JSON format implicit datacontenttype is asymmetrical

[jskeet]

Just trying to write tests for the clarifications in #861, and I think there's an asymmetry:

• When deserializing a CloudEvent, if datacontenttype isn't specified it defaults to application/json
• When serializing a CloudEvent, there's no such defaulting that I can see

I suspect that we should say that if datacontenttype is missing when serializing - and if data is present - it should default to application/json, so the payload should be serialized as JSON.

I'll proceed along those lines, and we can discuss whether that was actually the intention - and I can put together a PR to clarify the spec if so.

[jskeet]

(I may well have just missed it, of course...)

cc @dazuma

[jskeet]

(Just in case I forget - Clemens pointed out that the core spec does say that an event using JSON format can be implicitly application/json. So I'll just make that absolutely concrete in 3.1.1)

[manuelstein]

regarding that the addition refers to the user-provided datacontenttype plainly as "the datacontenttype", I noticed it's introduced in above (see 3.1) as "the datacontenttype attribute", so referring to it as "the datacontenttype" would be ok for me, but if you want to be clear, could use "the datacontenttype attribute". Would that clear things up? @duglin @jskeet

[duglin]

@jskeet can we close this now?

[jskeet]

Yup!