set response body to actually send JSON back

[antstanley]

Content-Type set to 'application/json', but a string is being sent back. Update to send back a JSON object.

TODO:

• Update CHANGELOG.md with your change (include reference to issue & this PR)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass

[abernix]

Thanks for opening this originally, and sorry we didn't get to this sooner. I've gone ahead and cleaned up and merged #753 which was a different take on this which instead utilizes Azure Functions' isRaw attribute, which should have a performance advantage over using JSON.parse for an object which will ultimately be re-stringified before going over the wire.

(I've posted this similar comment to #885, which was another attempt at the same change.)