title: Angular2 form control valueChanges observable complete never called
body: <p>I'm currently trying to show a simple loader on my search bar while the searching is taking place. I have planned to set a variable in the subscribe callback on the <code>valueChanges</code> observable from my form control to a value "loading", and to set it to an empty string in the complete callback. However, the complete callback is never called. </p>
  
  <p>I have also tried adding a callback to finally on the observable, but it is also never called.</p>
  
  <p>My code:</p>
  
  <pre><code>searchBox: Control = new Control();
  loadingClass: string = "";
  
  constructor() {
      this.searchBox.valueChanges
              .debounceTime(400)
              .distinctUntilChanged()
              .subscribe((text: string) =&gt; {
                  this.imageSearch = text;
                  this.loadingClass = "loading";
              }, (err: Error) =&gt; {
                  console.log(err);
              }, () =&gt; {
                  this.loadingClass = "";
                  console.log("test");
              });
  }
  </code></pre>