title: observer gets default value when subscribed
body: <p>I have a Subject that is happily emitting values to subscribes.</p>
  
  <p>I wonder, is it possible to get default value when you subscribe to the observable? Observable is fixed in this case, and observers are getting added on the fly. But when added, it should receive default value.</p>
  
  <pre><code>var subject = new Rx.Subject("default"); //sth like this
  subject.onNext("first value");
  
  var newSubscriberHere = subject.subscribe(function(val){
    console.log(val);  // should print the default value.
  });
  </code></pre>