fix internal function indexOf, when undefined passed

select2.js

@@ -70,6 +70,10 @@
     function indexOf(value, array) {
         var i = 0, l = array.length, v;
 
+        if (typeof value == 'undefined') {
+          return -1;
+        }
+
         if (value.constructor === String) {
             for (; i < l; i = i + 1) if (value.localeCompare(array[i]) === 0) return i;
         } else {