fix; correctly handle changes in update hook (Fix #3549)

Author: vkarpov15

lib/query.js

@@ -1875,21 +1875,21 @@ function convertSortToArray(opts) {
 }
 
 /**
- * Internal helper for update
+ * Internal thunk for .update()
  *
- * @param {Object} castedQuery
- * @param {Object} castedDoc the update command
- * @param {Object} options
  * @param {Function} callback
- * @return {Query} this
  * @see Model.update #model_Model.update
  * @api private
  */
-Query.prototype._execUpdate = function(castedQuery, castedDoc, options, callback) {
+Query.prototype._execUpdate = function(callback) {
   var schema = this.model.schema;
   var doValidate;
   var _this;
 
+  var castedQuery = this._conditions;
+  var castedDoc = this._update;
+  var options = this.options;
+
   if (this._castError) {
     callback(this._castError);
     return this;
@@ -2088,7 +2088,7 @@ Query.prototype.update = function(conditions, doc, options, callback) {
 
   // Hooks
   if (callback) {
-    return this._execUpdate(castedQuery, castedDoc, options, callback);
+    return this._execUpdate(callback);
   }
 
   return Query.base.update.call(this, castedQuery, castedDoc, options, callback);