title: ngOnChange not storing previousValue property
body: <p>[Angular2 @ RC4] [angularfire2 @ 2.0.0-beta.2]</p>
  
  <p>Inside my child component I cannot get <code>changes['posX'].previousValue</code> to store anything.</p>
  
  <p>Snippet from parent html:</p>
  
  <pre><code>//inside *ngfor loop
  [posX]='coord.position_x | calculateX:395'
  [posY]='coord.position_y | calculateY:380'
  </code></pre>
  
  <p>Snippet from Child component:</p>
  
  <pre><code>export class childComponent implements OnChanges {
    @Input() posX: number; //updated via firebase database from parant
    @Input() posY: number; //updated via firebase database from parant
  
    ngOnChanges(changes: {[ propName: string]: SimpleChange}) {
      console.log('Change detected: ', changes['posX'].currentValue);
      console.log('Change detected: ', changes['posX'].previousValue);
    }
  </code></pre>
  
  <p>Result from chrome console :</p>
  
  <pre><code>Change detected: posX current =  343 //changes every time
  Change detected: posX previous = Object {} //same after updates
  </code></pre>
  
  <p>I found this <a href="http://plnkr.co/edit/AHrB6opLqHDBPkt4KpdT?p=preview" rel="nofollow">plnkr</a> of ngOnChange and was able to get it working in my app. Although I'm unable to get the PosX and PosY variables working the same way. </p>
  
  <p>The Variables PosX and PosY are being updated via an observable from my firebase database every 15 seconds.  Any idea why previousValue is not showing up?</p>