title: firebase snapshot.downloadURL is undefined after successfully uploading an image
body: <p>I have this code in my app, that I use to upload an image and get its url so that i can save it in the database, the image is in base64 format and the upload is successfull as i can see in <code>console.log(snapshot);</code> output, and by checking also in my firebase storage however, the downloadUrl property of snapshot is undefined i dont know why.  This is not the way it was supposed to work</p>
  
  <pre><code>storage.$putString(b64, 'data_url', {contentType:'image/jpg'}).$complete(function(snapshot) {
          console.log(snapshot);
          item.avatarUrl=snapshot.downloadURL;
          agents.$add(item).then(function(ref) {
          });
  
      });     
  </code></pre>