[OAS 3.1.0] Operation.requestBody now permitted for HTTP methods which don't allow a request body

[Azquelt]

Operation.requestBody now allows a body to be provided even if the HTTP method of the operation doesn't support it (e.g. GET).

We may want to:

• Update any relevant documentation
• Add a test for a GET, HEAD and DELETE operations with a request body specified

[Azquelt]

Can't see anywhere that we document the old restriction, so this just needs a new test.

[benjamin-confino]

Please assign this to me.