`HttpClient` can re-subscribe and re-feed the request body in certain scenarios. Plus, [the Reactive Streams specification] explicitly states this as a rule: "Publisher.subscribe MAY be called as many times as wanted". This is sometimes informally referred as "replayable publishers" too. We need to ensure this property holds for all request body publishers.

[the Reactive Streams specification]: https://github.com/reactive-streams/reactive-streams-jvm