Simplify collection initialization (#6897)

- Addresses VS message "IDE0028 Collection initialization can be simplified"

Author: mikeharder

test/Microsoft.AspNetCore.Mvc.FunctionalTests/BasicTests.cs

@@ -406,8 +406,10 @@ public async Task UsingPageRouteParameterInAttributeRouteWorks()
         public async Task RedirectToAction_WithEmptyActionName_UsesAmbientValue()
         {
             // Arrange
-            var product = new List<KeyValuePair<string, string>>();
-            product.Add(new KeyValuePair<string, string>("SampleInt", "20"));
+            var product = new List<KeyValuePair<string, string>>
+            {
+                new KeyValuePair<string, string>("SampleInt", "20")
+            };
 
             // Act
             var response = await Client.PostAsync("/Home/Product", new FormUrlEncodedContent(product));