Use "set" instead of "splice" for child changed

When using firebase-collection in a dom-repeat, the whole template has to refresh every time a child changes instead of only the modified child. That's because it is updating the array with a splice.

If we use a set instead, only the actual child is refreshed. The main problem with the "splice" was that I was losing focus on an input when the refresh was happening.

*fixed coding style

Author: louisptremblay

firebase-collection.html

@@ -586,7 +586,7 @@ <h4>[[dinosaur.__firebaseKey__]]</h4>
 
           this._valueMap[oldValue.__firebaseKey__] = null;
           this._valueMap[value.__firebaseKey__] = value;
-          this.splice('data', this.data.indexOf(oldValue), 1, value);
+          this.set('data.' + this.data.indexOf(oldValue), value);
         });
       },