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.

Individua il codice di Spring CloudEventMessageConverter in sdk-spring e verifica come legge gli header content-type. Verifica l’interazione tra serializzatore e deserializzatore descritta nell’issue, quindi mantieni i fallback esistenti supportando al contempo il nome dell’header della specifica CloudEvents; il lavoro è completato quando i messaggi strutturati da sdk-kafka vengono deserializzati tramite sdk-spring.