message: prepare should be idempotent
diff: @@ -367,7 +367,7 @@ impl<
  let envelope = Envelope::new_req_prepare(query.to_string(), keyspace, flags, self.version);
  - self.send_envelope(envelope, false, None, None, None, None, None, None)
  + self.send_envelope(envelope, true, None, None, None, None, None, None)
  .await
  .and_then(|response| response.response_body())
  .and_then(|body| {
  