fix(DataSet): make sure the right id-type is used internaly

This fixes a fixed strange string-number conversion issue.
It can happen that id "0" !== 0 what causes #2580.

fixes #2580

examples/graph2d/08_performance.html

@@ -147,4 +147,4 @@
     var graph2d = new vis.Graph2d(container, dataset, options);
 </script>
 </body>
-</html>
+</html>

lib/DataSet.js

@@ -730,7 +730,7 @@ DataSet.prototype.clear = function (senderId) {
   var i, len;
   var ids = Object.keys(this._data);
   var items = [];
-  
+
   for (i = 0, len = ids.length; i < len; i++) {
     items.push(this._data[ids[i]]);
   }
@@ -912,7 +912,7 @@ DataSet.prototype._getItem = function (id, types) {
   }
 
   if (!converted[this._fieldId]) {
-    converted[this._fieldId] = id;
+    converted[this._fieldId] = raw.id;
   }
 
   return converted;