title: RxJava2 concat() runs only the first Observable
body: <p>I have 3 methods which return <code>Observable(String)</code>. I need to perform this methods one by one. For each method neccessary approximately 2 or 3 minutes. I am using the next code:</p>
  
  <pre><code>Observable.concat(one(),two(),three())
            .subscribeWith(new DisposableObserver&lt;String&gt;() {
                      @Override
                      public void onNext(String s) {
                          LOG.debug(s);
                      }
  
                      @Override
                      public void onError(Throwable e) {
                          LOG.error(e.toString());
                      }
  
                      @Override
                      public void onComplete() {
  
                      }
                  });
  </code></pre>
  
  <p>Only the first method is performed. The second and the third method are ignored. I can't understand what is the problem? I am using RxJava2 version 2.1.6</p>