Use new GetContentTypeDescriptor() overload in Save()

The new `GetContentTypeDescriptor()` method (408d01a) provides a cleaner way of getting the associated `ContentTypeDescriptor` from the current topic, while simultaneously offering a fallback of retrieving missing `ContentTypeDescriptor`s from the in-memory topic graph if they can't be found in the `GetContentTypeDescriptors()` cache.

Author: JeremyCaney

OnTopic/Repositories/TopicRepositoryBase.cs

@@ -292,7 +292,7 @@ public virtual int Save([ValidatedNotNull, NotNull]Topic topic, bool isRecursive
       | Validate content type
       \-----------------------------------------------------------------------------------------------------------------------*/
       var contentTypeDescriptors= GetContentTypeDescriptors();
-      var contentTypeDescriptor = contentTypeDescriptors.GetTopic(topic.ContentType);
+      var contentTypeDescriptor = GetContentTypeDescriptor(topic);
 
       if (contentTypeDescriptor == null) {
         throw new ArgumentException(