Overriding `request.idempotent?` for individual requests

[jakeonfire]

we have some service connections where activating the built-in async-http retry behavior makes sense for idempotent requests, and we have built a module-prepend monkey patch to override request.idempotent? for these cases (source). do you think formalizing an interface to do this as part of async-http would be worthwhile? happy to give a PR a stab if so.

originally we had added our own, surrounding retry behavior, but it seems more efficient to re-use async-https, the only blocker being that it is conditional on idempotency.

[jakeonfire]

there was a flaw in my plan, as request bodies are not rewound. we're removing the monkey-patch and using outer retries in our client code instead.