title: distinctUntilChanged() on BehaviourSubject don't work
body: <p>My package.json:</p>
  
  <ul>
  <li>rxjs": "^5.5.6"</li>
  <li>@angular: "6.0.0-beta.5</li>
  </ul>
  
  <p><code>searchTextValueSubject</code> is a BehaviorSubject</p>
  
  <pre><code>import { debounceTime } from 'rxjs/operators';
  import { distinctUntilChanged } from 'rxjs/operators';
  import { pipe } from 'rxjs';
  
  this.sharingVariableService
      .searchTextValueSubject
      .pipe(
          distinctUntilChanged((x, y) =&gt; {
              console.log(y.BarCodeS === x.BarCodeS);
              console.log(y.BarCodeS);
              console.log(x.BarCodeS);
              return y.BarCodeS === x.BarCodeS
          })
      )
      .subscribe((searchTextValue) =&gt; {
          this.searchTextValue = searchTextValue;
          this.getRessourceHardware(this.serverDataRessourceHardware._meta.max_results, this.searchTextValue);
      });
  </code></pre>
  
  <p>Result :</p>
  
  <blockquote>
    <p>true, 'd', 'd'</p>
    
    <p>true, 'da', 'da'</p>
  </blockquote>
  
  <p>Expected :</p>
  
  <blockquote>
    <p>False, ' ', 'd'</p>
    
    <p>False, 'd', 'da'</p>
  </blockquote>
  
  <p>I probably don't understand something. Could you please point me to the right direction ?</p>