Fixed specific type of NotFoundResult in unit test

When content is included in the `NotFound` result, the `Controller` class returns a `NotFoundObjectResult`. When no content is included, however, it returns a `NotFoundResult`. As such, when I upaded the `[ValidateTopic]` action filter to return `NotFound()` without a specific error message (ab71a69), I also needed to update the `ValidateTopicAttributeTest` to correctly expect the `NotFoundResult` instead of the `NotFoundObjectResult`. Whoops.

Author: JeremyCaney

OnTopic.AspNetCore.Mvc.Tests/ValidateTopicAttributeTest.cs

@@ -111,7 +111,7 @@ public void InvalidControllerType_ThrowsException() {
     | TEST: NULL TOPIC: RETURNS NOT FOUND
     \-------------------------------------------------------------------------------------------------------------------------*/
     /// <summary>
-    ///   Ensures that a <see cref="NotFoundObjectResult"/> is thrown if the <see cref="TopicController.CurrentTopic"/> is null.
+    ///   Ensures that a <see cref="NotFoundResult"/> is thrown if the <see cref="TopicController.CurrentTopic"/> is null.
     /// </summary>
     [Fact]
     public void NullTopic_ReturnsNotFound() {
@@ -124,7 +124,7 @@ public void NullTopic_ReturnsNotFound() {
 
       controller.Dispose();
 
-      Assert.IsType<NotFoundObjectResult>(context.Result);
+      Assert.IsType<NotFoundResult>(context.Result);
 
     }