Updated unit test to pass root content type descriptor

Previously, the `GetContentTypeDescriptors_WithTopicGraph_ReturnsMergedContentTypes()` unit test validted its functionality by passing in a specific content type. This worked, but only because the initial functionality failed to validate that the `ContentTypeDescriptor` being sent was, in fact, the expected root `ContentTypeDescriptor`. This issue will ultimately be fixed by using the new `ContentTypeDescriptorCollection.Refresh()` method (3e3930f)'s validation (b206f3e). In anticipation of migrating to that, the unit test is being updated to be more specific.

Author: JeremyCaney

OnTopic.Tests/TopicRepositoryBaseTest.cs

@@ -321,7 +321,7 @@ public void GetContentTypeDescriptors_WithTopicGraph_ReturnsMergedContentTypes()
       var newContentType        = TopicFactory.Create("NewContentType", "ContentTypeDescriptor", rootContentType);
       var contentTypeCount      = contentTypes.Count;
 
-      _topicRepository.GetContentTypeDescriptorsProxy((ContentTypeDescriptor)newContentType);
+      _topicRepository.GetContentTypeDescriptorsProxy(rootContentType);
 
       Assert.AreNotEqual<int>(contentTypeCount, contentTypes.Count);
       Assert.IsNotNull(contentTypes.Contains(newContentType));