* Fix panic in async wait() method when RequiredAcks is None

When RequiredAcks is None, the producer does not wait for a
response from the broker, therefore the response is nil.
The async wait() method was not handling this case, leading
to a panic.

* Add regression test for RequiredAcks == RequireNone

This new test is required because all the other Writer tests use
NewWriter() to create Writers, which sets RequiredAcks to
RequireAll when 0 (None) was specified.