HTTP/2 stream cancelImpl may leave subscriber registered

Some of the ThrowingSubscriberXxx tests (typically those that use a streaming body) have been observed failing intermittently (though rarely) in timeout. The error message printed at the end showed that some HTTP/2 stream subscribers had not been properly deregistered when an exception was thrown in GET_BODY.

The issue appears to be a race condition between cancelImpl and schedule where the scheduler `sched` may get stopped before the subscriber is exceptionally completed.

A simple fix is to ensure that cancelImpl will complete the subscriber, if needed.