Allow conversion to JsonObject to be optional during amqp message deserialization

[ylepikhov]

Documentation states: The connector converts incoming AMQP Messages into Reactive Messaging Message<T> instances. T depends on the body of the received AMQP Message. In case of AMQP Data (with binary content) and the content-type is set to application/json T becomes JsonObject. It might be unwanted. One might want to leave payload as is (without any deserialization/serialization), or use alternative deserialization (Jackson, etc). It will be nice to have a configuration option for AmqpConnetor to disable conversion to JsonObject in case of json content type.