function updateWithConfig( obj, config ) {
        if (config) {
            assertConfig(config)
                .whereParam("fetchStrategy").isEnumOf(FetchStrategy).isOptional()
                .whereParam("mergeStrategy").isEnumOf(MergeStrategy).isOptional()
                .applyAll(obj);
        }
        return obj;
    }
Sets the 'defaultInstance' by creating a copy of the current 'defaultInstance' and then applying all of the properties of the current instance. 
    The current instance is returned unchanged.
    @method setAsDefault
    @example
        var newQo = new QueryOptions( { mergeStrategy: MergeStrategy.OverwriteChanges });
        newQo.setAsDefault();
    @chainable