Switching to NuGet Ardalis.Specifications (#389)

- updating to use Ardalis.Specifications package as it is maintained and has a more robust implementation
- Removing all custom specification implementation
- Updating unit tests

src/ApplicationCore/Specifications/BasketWithItemsSpecification.cs

@@ -1,16 +1,15 @@
-using Microsoft.eShopWeb.ApplicationCore.Entities.BasketAggregate;
+using Ardalis.Specification;
+using Microsoft.eShopWeb.ApplicationCore.Entities.BasketAggregate;
 
 namespace Microsoft.eShopWeb.ApplicationCore.Specifications
 {
     public sealed class BasketWithItemsSpecification : BaseSpecification<Basket>
     {
-        public BasketWithItemsSpecification(int basketId)
-            :base(b => b.Id == basketId)
+        public BasketWithItemsSpecification(int basketId) : base(b => b.Id == basketId)
         {
             AddInclude(b => b.Items);
         }
-        public BasketWithItemsSpecification(string buyerId)
-            :base(b => b.BuyerId == buyerId)
+        public BasketWithItemsSpecification(string buyerId) : base(b => b.BuyerId == buyerId)
         {
             AddInclude(b => b.Items);
         }