Fix DMF docs and unnecessary content_type enforcements. (#792)

* Fix DMF docs and unnecessary content_typ enforcements.

* One more error.

* Typo

hawkbit-dmf/hawkbit-dmf-amqp/src/main/java/org/eclipse/hawkbit/amqp/AmqpMessageHandlerService.java

@@ -132,7 +132,6 @@ public class AmqpMessageHandlerService extends BaseAmqpService {
             final MessageType messageType = MessageType.valueOf(type);
             switch (messageType) {
             case THING_CREATED:
-                checkContentTypeJson(message);
                 setTenantSecurityContext(tenant);
                 registerTarget(message, virtualHost);
                 break;