Migrated some tests to use System.Text.Json (#385)

* Migrated some tests to use System.Text.Json

* Added case insensitive json options to fix broken tests.

src/Web/ViewModels/CatalogIndexViewModel.cs

@@ -5,9 +5,9 @@ namespace Microsoft.eShopWeb.Web.ViewModels
 {
     public class CatalogIndexViewModel
     {
-        public IEnumerable<CatalogItemViewModel> CatalogItems { get; set; }
-        public IEnumerable<SelectListItem> Brands { get; set; }
-        public IEnumerable<SelectListItem> Types { get; set; }
+        public List<CatalogItemViewModel> CatalogItems { get; set; }
+        public List<SelectListItem> Brands { get; set; }
+        public List<SelectListItem> Types { get; set; }
         public int? BrandFilterApplied { get; set; }
         public int? TypesFilterApplied { get; set; }
         public PaginationInfoViewModel PaginationInfo { get; set; }