Focus is lost when the change event is triggered

This closes https://github.com/select2/select2/issues/3332.

src/js/select2/selection/search.js

@@ -128,6 +128,8 @@ define([
   };
 
   Search.prototype.update = function (decorated, data) {
+    var searchHadFocus = this.$search[0] == document.activeElement;
+
     this.$search.attr('placeholder', '');
 
     decorated.call(this, data);
@@ -136,6 +138,9 @@ define([
                    .append(this.$searchContainer);
 
     this.resizeSearch();
+    if (searchHadFocus) {
+      this.$search.focus();
+    }
   };
 
   Search.prototype.handleSearch = function () {