title: Prism Unsubscribe with subscription token not working
body: <p>I am subscribing and unsubscribing to prism events using the code below in ClassA. The problem I am having is <strong>after</strong> I unsubscribe, and another completely different class, say ClassB, with a different handler registers for the same event, ClassA handler is still invoked. Why is this?</p>
  
  <p>I have tried both unsubscribing using a token as well as the method delegate used when registering, both to no avail.</p>
  
  <pre><code>SubscriptionToken _subscriptionToken;
  
  //register subscription + handler
  var pevent = GetEventAggregator().GetEvent&lt;PriceSubscriptionEvent&gt;();
  _subscriptionToken = pevent.Subscribe(r =&gt;
  {
      DataHandler(r);
      return;
  }, ThreadOption.BackgroundThread, false, null);
  
  //Unsubscribe
  var pevent = GetEventAggregator().GetEvent&lt;PriceSubscriptionEvent&gt;();
  pevent.Unsubscribe(_subscriptionToken);
  </code></pre>