[XMLHttpRequest] Clearing the readystatechange listener

The draft does not indicate what happens to the readystatechange listener when 
the request completes.  If another request is then dispatched using the same 
XMLHttpRequest object, does the readystatechange listener persist?

Gecko's behavior right now is to clear all event listeners (including 
readystatechange) when the request goes into readyState 4.  Not sure what other 
UAs do.

-Boris