Adding guards and more tests (#68)

* Adding single entity by spec method to repository

* Adding guards and more unit tests

src/ApplicationCore/Interfaces/IRepository.cs

@@ -1,12 +1,12 @@
 using Microsoft.eShopWeb.ApplicationCore.Entities;
 using System.Collections.Generic;
-using System.Threading.Tasks;
 
 namespace ApplicationCore.Interfaces
 {
     public interface IRepository<T> where T : BaseEntity
     {
         T GetById(int id);
+        T GetSingleBySpec(ISpecification<T> spec);
         IEnumerable<T> ListAll();
         IEnumerable<T> List(ISpecification<T> spec);
         T Add(T entity);