Don't listen to events coming from aborted XmlHTTPRequests

Created by: robertbryer

After aborting an event, the onloadend event fires still. This causes the aborted segment to look like an error and will be retried, and this causes a second scheduling loop to start up, leading to segments being downloaded multiple times concurrently (an example is in the logs in #2588 (closed) , but this occurs on all platforms).