fixed error caused by function in config.data_order

c3.js

@@ -1617,11 +1617,11 @@
     };
     c3_chart_internal_fn.isOrderDesc = function () {
         var config = this.config;
-        return config.data_order && config.data_order.toLowerCase() === 'desc';
+        return typeof(config.data_order) === 'string' && config.data_order.toLowerCase() === 'desc';
     };
     c3_chart_internal_fn.isOrderAsc = function () {
         var config = this.config;
-        return config.data_order && config.data_order.toLowerCase() === 'asc';
+        return typeof(config.data_order) === 'string' && config.data_order.toLowerCase() === 'asc';
     };
     c3_chart_internal_fn.orderTargets = function (targets) {
         var $$ = this, config = $$.config, orderAsc = $$.isOrderAsc(), orderDesc = $$.isOrderDesc();

src/data.js

@@ -206,11 +206,11 @@ c3_chart_internal_fn.hasPositiveValueInTargets = function (targets) {
 };
 c3_chart_internal_fn.isOrderDesc = function () {
     var config = this.config;
-    return config.data_order && config.data_order.toLowerCase() === 'desc';
+    return typeof(config.data_order) === 'string' && config.data_order.toLowerCase() === 'desc';
 };
 c3_chart_internal_fn.isOrderAsc = function () {
     var config = this.config;
-    return config.data_order && config.data_order.toLowerCase() === 'asc';
+    return typeof(config.data_order) === 'string' && config.data_order.toLowerCase() === 'asc';
 };
 c3_chart_internal_fn.orderTargets = function (targets) {
     var $$ = this, config = $$.config, orderAsc = $$.isOrderAsc(), orderDesc = $$.isOrderDesc();