Camel-8024 Improvement to camel-sjms JMSMessageHelper

Hi all,

This PR is related to: https://issues.apache.org/jira/browse/CAMEL-8024

Please let me know if something can be improved or if something is wrong. I've added some check on discoverMessageTypeFromPayload method and a new test class.

Bye,
Andrea
Need to assert that a TextMessage was created, and the type of class in Exchange.getIn().getBody() is String.class.