Don't retry POST request when it's already sent

[Yuras]

The library retries HTTP request when NoResponseDataReceived is thrown. But it's thrown after the entire request is already sent to server, and retrying is not safe since request might already reach the server.

I encountered the issue in the wild. Looks like AWS load balancer closes connection after inactivity (idle timeout cutoff) without sending anything to the client. As a result, if the connection was reused, http-client retries the request. Probably our AWS load balancer is misconfigured, but arguably http-client should not retry is such cases. If it's not possible to fix (e.g. because of backward-compatibility concerns), then consider documenting the behavior.