Explicitly state application/json defaulting when serializing
Explicitly state application/json defaulting when serializing

(I've used SHOULD rather than MUST to be consistent with deserialization. If we were starting from scratch, I'd suggest that MUST would be more appropriate, but it's probably too late to do that.)

Fixes #880

Signed-off-by: Jon Skeet <8a9602d66334021c18cbf0bdb2fa15f4cb6de9df@google.com>

cloudevents/spec