A bug in apply(u:=>Unit) method of Subscription companion object fixed w...

...hich may cause multiple invocations of u, in a multithreaded, multi-unsubscribe situation.

Threads may be pre-empted after evaluation of condition (!unsubscribed.get()) which may result in two threads executing the consequent block more than once. A code that relies on single evaluation of u will fail if that case happens.