Add more tests for options.allowGetBody

[PopGoesTheWza]

To help investigate #1186 this PR adds:

• withBodyParsingServer helper to allow the use of request.body in test cases
• two test cases for using allowGetBody with a json payload

Adding more allowGetBody with non-json payloads might be relevant but not in the current scope of originating issue.

Checklist

• I have read the documentation.
• I have included a pull request description of my changes.
• I have included some tests.
• If it's a new feature, I have included documentation updates.

[szmarczak]

Thanks for submitting! 😃