title: empty ng-src doesn't update image
body: <p>I am using the <strong>ng-src</strong> directive in order to avoid the browser to request the image before Angular evaluate the expression.</p>
  
  <p><code>ng-src={{image}}</code> will update the <em>src</em> attribute of the image if the expression "image" change. 
  I misunderstand why the <em>ng-src</em> directive doesn't update the path of the image if the expression ("myImage.png") become empty ("").</p>
  
  <p>When the expression become empty, the <em>ng-src</em> attribute become empty but the <em>src</em> attribute still the last known <em>src</em>. So it doesn't update the image. Why the <em>src</em> attribute isn't updated (to an empty <em>src</em>) so that the image "disappear".</p>
  
  <p>Here is a <a href="http://plnkr.co/edit/eYk9je?p=preview">plunker </a></p>
  
  <p>Thanks</p>