incorrectly calling Kernel.sleep when producers are created with max_send_retries set to 0

I'm using poseidon in a project which we wanted to implement our own failure logic.  I noticed though that if I set max_send_retries to 0 it would still wait 100ms unless I also set retry_backoff to 0 as well.  If max_send_retries is set to 0, the retry_backoff shouldn't matter.