As documented: OnRecv causes all requests received on conn to invoke
f(req, nil) and all responses to invoke f(req, resp).
Since OnRecv is called with both *Request and *Response being non-nil
when we're handling a response, we need to check that *Response is
non-nil before we check *Request is non-nil. This change just swaps the
two cases in the switch statement to fix the issue. For consistency,
I've swapped the cases for OnSend also, even when it's not needed.
Before this commit, a custom logger could be set with the LogMessages
function. However, by using LogMessages not only is a custom logger set
but also all received and sent messages are logged. Use cases exist
where a custom logger is desired to log errors but not all messages.
