Spring CloudEventMessageConverter content-type header name

Spring CloudEventMessageConverter content-type header name uses the Spring Messaging contentType header name which differs fom the cloud events spec content-type which is used by Kafka serializer for instance.

This leads to a message from a project using structured mode and kafka serializer (sdk-kafka) to not be deserialized by another project using (sdk-spring).

The current code is as follows on sdk-spring:

        if (message.containsKey(MessageHeaders.CONTENT_TYPE)) {
            return String.valueOf(message.get(MessageHeaders.CONTENT_TYPE));
        }
        if (message.containsKey(CloudEventsHeaders.CONTENT_TYPE)) {
            return String.valueOf(message.get(CloudEventsHeaders.CONTENT_TYPE));
        }

Unfortunately the CloudEventsHeaders.CONTENT_TYPE is CE_PREFIX + "datacontenttype" which is not exactly the same property we are looking here.

I would be happy to create a merge request where a another fallback is introduced to have the spec header name (content-type) which would enable the sdk-spring to deserialize messages.

This would add the support while still keeping it compatible with different versions and services.

Ideally, also the spring-sdk serializer should be checked and changed, but i would rather have this small mr fixed and dive on the serializer in a different issue.

Locate the Spring CloudEventMessageConverter code in sdk-spring and inspect how it reads content-type headers. Verify the serializer and deserializer interaction described in the issue, then preserve the existing fallbacks while supporting the CloudEvents spec header name; done means structured messages from sdk-kafka deserialize through sdk-spring.