title: How to get a Disposable for subjects as subscriber in RxJava2
body: <p>It might be a silly question, but how do I get a <code>Disposable</code> when subscribing a <code>Subject</code> to an Observable in RxJava 2.0?</p>
  
  <p>For example:</p>
  
  <pre><code>observable.subscribeWith( behaviorSubject)
  </code></pre>
  
  <p>doesn't return a <code>Disposable</code>? How do I cancel such a subscription?</p>
  
  <p>Or another example with <code>CompositeDisposable</code>:</p>
  
  <pre><code>compositeDisposable.add( observable.subscribeWith( behaviorSubject) ) ) 
  </code></pre>
  
  <p>This doesn't compile because <code>subscribeWith( behaviorSubject )</code> doesn't return a Disposable.</p>
  
  <p>How do I unsubscribe / dispose / cancel properly with Subjects?</p>