团购：默认curDiscount值

shared/domain/activityField/groupShopField.ts

@@ -81,6 +81,7 @@ class GroupShopItem {
         if(item.discount && item.discount.length > 0) {
             for(let discount of item.discount) {
                 this.discount.push(new GroupShopDiscount(item.price, discount));
+                if(!this.curDiscount) this.curDiscount = new GroupShopDiscount(item.price, discount);
             }
         }
     }