title: Make Any Observable act like ReplaySubject
body: <p>I've been attempting to get my stream to replay only the latest value with little luck. Basically, I've got a <code>replaySubject(1)</code> in a place where I can't just manually <code>.onNext</code> it. What I would like to do is take it, map it to something else, then add an initial value.</p>
  
  <p>My first instinct was the following:</p>
  
  <pre><code>var testInStream = new Rx.ReplaySubject(1);
  var testOutStream = testInStream.startWith(-1);
  
  testInStream.onNext(0);
  
  setTimeout(function(){
         testOutStream.subscribe(function(x){ 
         console.log("stream:", x);
     });
  }, 1000);
  </code></pre>
  
  <p>Which prints</p>
  
  <pre><code>stream: test -1
  stream: test 0
  </code></pre>
  
  <p>However, I would like to find a way to only print <code>stream: test 0</code>. The closest thing I can come up with is</p>
  
  <pre><code>var testInStream = new Rx.ReplaySubject(1);
  var testOutStream = testInStream.startWith(-1);
  
  testInStream.onNext(0);
  
  var wrapperSubject = new Rx.ReplaySubject(1);
  testOutStream.subscribe(function(new_val){ wrapperSubject.onNext(new_val) });
  
  setTimeout(function(){
    wrapperSubject.subscribe(function(x){ 
      console.log("stream:", x);
    });
  }, 1000);
  </code></pre>
  
  <p>Which returns</p>
  
  <pre><code>stream: test 0
  </code></pre>
  
  <p>Which is what I want. But, I would rather not create an intermediate replaySubject, I would prefer to just call something like <code>testOutStream.latest()</code> or <code>testOutStream.replay(1)</code> but I've yet to get any of them to work.</p>
  
  <p>Any help is appreciated, here's a <a href="https://jsbin.com/hiketa/edit?js,console" rel="nofollow">JS Bin</a> with my examples.</p>