[GO-2023-2328] HTTP request body disclosure in github.com/go-resty/resty/v2

A race condition in go-resty can result in HTTP request body disclosure across
requests.

This condition can be triggered by calling sync.Pool.Put with the same
*bytes.Buffer more than once, when request retries are enabled and a retry
occurs. The call to sync.Pool.Get will then return a bytes.Buffer that hasn't
had bytes.Buffer.Reset called on it. This dirty buffer will contain the HTTP
request body from an unrelated request, and go-resty will append the current
HTTP request body to it, sending two bodies in one request.

The sync.Pool in question is defined at package level scope, so a completely
unrelated server could receive the request body.