fix some issue and cover by test (#669)

src/Web/Controllers/ManageController.cs

@@ -1,8 +1,5 @@
-using System;
-using System.Linq;
-using System.Text;
+using System.Text;
 using System.Text.Encodings.Web;
-using System.Threading.Tasks;
 using Microsoft.AspNetCore.Authentication;
 using Microsoft.AspNetCore.Authorization;
 using Microsoft.AspNetCore.Identity;

src/Web/Controllers/OrderController.cs

@@ -1,5 +1,4 @@
-using System.Threading.Tasks;
-using MediatR;
+using MediatR;
 using Microsoft.AspNetCore.Authorization;
 using Microsoft.AspNetCore.Mvc;
 using Microsoft.eShopWeb.Web.Features.MyOrders;