message: fix(service-worker): fix improper call of Observable.merge
  Observable.merge was called using .call() as if it were an operator
  and not an Observable factory. This removes the .call() and uses
  the factory properly.
  PR Close
diff: @@ -43,7 +43,7 @@ export class SwPush {
  const workerDrivenSubscriptions = <Observable<PushSubscription|null>>(op_switchMap.call(
  this.pushManager, (pm: PushManager) => pm.getSubscription().then(sub => { return sub; })));
  - this.subscription = obs_merge.call(workerDrivenSubscriptions, this.subscriptionChanges);
  + this.subscription = obs_merge(workerDrivenSubscriptions, this.subscriptionChanges);
  }
  requestSubscription(options: {serverPublicKey: string}): Promise<PushSubscription> {
  