Fix caching previousShouldFire in pausableBuffered

Assigning `previousShouldFire = results.shouldFire;` must be done immediately.

Otherwise the call to `observer.onNext` can fire another call to the 
`subscribe` callback before `previousShouldFire` was assigned creating an erroneous result in
`if (previousShouldFire !== undefined && results.shouldFire != previousShouldFire)`

Fixes https://github.com/Reactive-Extensions/RxJS/issues/277