INT-3535: Fix `JmsHM` to not override properties

JIRA: https://jira.spring.io/browse/INT-3535

Previously when `DefaultJmsHeaderMapper` overrode values, which had been able to be populated by the `MessageConverter`.

Since `MessageConverter` result has a precedence which is closer to JMS, skip those custom `MessageHeaders` which already has been populated by `MessageConverter`