Fix MessagingMessageConverter to handle JSON string payload configured with the APPLICATION_JSON header (#1144)

[imsosleepy]

:loudspeaker: Type of change

• [ ] Bugfix
• [ ] New feature
• [x] Enhancement
• [ ] Refactoring

:scroll: Description

When creating a raw JSON string type payload in the SQS Template, if the header is set to CONTENT_TYPE as APPLICATION_JSON and sent to SQS, the listener could not read the object in the same format.

:bulb: Motivation and Context

Implementation for issue #1144

:green_heart: How did you test it?

:pencil: Checklist

• [x] I reviewed submitted code
• [x] I added tests to verify changes
• [ ] I updated reference documentation to reflect the change
• [x] All tests passing
• [x] No breaking changes

:crystal_ball: Next steps