Suspends the current request. This may have the effect of closing any underlying transport (in order to free up resources), although any open streams remain logically opened and will continue delivering data when the transport is resumed.
Calling cancel() on a suspended request must not send any notifications (such as onstopRequest) until the request is resumed.
NOTE: some implementations are unable to immediately suspend, and may continue to deliver events already posted to an event queue. In general, callers should be capable of handling events even after suspending a request.