Fallback to topic graph is GetAttributes() can't find AttributeDescriptor

Ideally, all `AttributeDescriptor`s will be available via as a result of the recent updates which update the cache of both `GetContentTypeDescriptors()` (a9ea634) and `ContentTypeDescriptor.AttributeDescriptors` (b0c5a5e) on `Save()`, `Delete()`, and `Move()`.

When doing recusive `Save()`s of large topic graphs—as is common when importing data using the new **OnTopic Data Exchange**—it is possible, however, that the `AttributeDescriptors` cache will be out-of-date if a `ContentTypeDescriptor` has been updated in memory, but not persisted at the time that a dependency `Topic` tries to implement it.

We face a similar issue with content types, and thus update the `GetContentTypeDescriptors()` reactively in `GetAttributes()` and `GetUnmatchedAttributes()` (408d01a). We can do something similar for `AttributeDescriptors` in `GetAttributes()`.

This satisfies the attributes-related requirements of the newly proposed feature enhancement: Discover in-memory ContentTypeDescriptor, AttributeDescriptor on Save() (#18).

Author: JeremyCaney

OnTopic/Repositories/TopicRepositoryBase.cs

@@ -492,6 +492,12 @@ protected IEnumerable<AttributeValue> GetAttributes(Topic topic, bool? isExtende
         var key                 = attributeValue.Key;
         var attribute           = (AttributeDescriptor?)null;
 
+        //Reset cached attribute descriptors just in case a new attribute has been added
+        if (!contentType.AttributeDescriptors.Contains(key)) {
+          contentType.ResetAttributeDescriptors();
+        }
+
+        //Attempt to retrieve the corresponding attribute descriptor
         if (contentType.AttributeDescriptors.Contains(key)) {
           attribute             = contentType.AttributeDescriptors[key];
         }