Utilize new IsDirty() method for evaluating byline, dateline

Instead of manually defining `isDirty` based on a custom LINQ expression, we can now just use the new `AttributeValueCollection.IsDirty()` method introduced in the forthcoming version of the **OnTopic Library** (80fee57).

Technically, because the same update of the library _also_ includes new code in the `TopicRepositoryBase` and e.g. `SqlTopicRepository` for bypassing the byline (`LastModifiedBy`) and dateline (`LastModified`) if no other attributes have been modified, we could just remove this condition entirely.

As this is a cheap condition to validate, however, there remains value to retaining this  condition, as it prevents the in-memory copies of the topics from being updated with the byline and dateline. That mitigates confusion of any pages displaying this information—including the **OnTopic Editor**—from being temporarily updated, even though the data won't be persisted to the repository.

In addition, it avoids a rare scenario where another process subsequently modifies one or more attribute on the in-memory topic and `Save()`s it, in which case the dateline and byline would then get persisted, even though they weren't related to that operation. In practice, most subsequent updates would be done by the **OnTopic Editor**, which will always update these attributes with information about the current edit, so this remains a tail risk.

Regardless, by keeping the `IsDirty()` condition in place, we prevent either of those scenarios, even if they're rare, while introducing minimal overhead.

Author: JeremyCaney

OnTopic.Data.Transfer/Interchange/TopicExtensions.cs

@@ -207,17 +207,10 @@ public static void Import(this Topic topic, TopicData topicData, [NotNull]Import
         );
       }
 
-      /*------------------------------------------------------------------------------------------------------------------------
-      | Determine changes
-      \-----------------------------------------------------------------------------------------------------------------------*/
-
-      //Defermine if any attributes have changed
-      var isDirty               = topic.Attributes.Any(a => a.IsDirty);
-
       /*------------------------------------------------------------------------------------------------------------------------
       | Handle special rules for LastModified(By) attribute
       \-----------------------------------------------------------------------------------------------------------------------*/
-      if (isDirty) {
+      if (topic.Attributes.IsDirty()) {
 
         switch (options.LastModifiedStrategy) {
           case LastModifiedImportStrategy.Current:
@@ -237,14 +230,14 @@ public static void Import(this Topic topic, TopicData topicData, [NotNull]Import
             break;
         }
 
-        if (topic.Attributes.GetValue("LastModifiedBy", null) == null) {
-          topic.Attributes.SetValue("LastModifiedBy", options.CurrentUser);
-        }
-
         if (topic.Attributes.GetValue("LastModified", null) == null) {
           topic.Attributes.SetValue("LastModified", DateTime.Now.ToString(CultureInfo.CurrentCulture));
         }
 
+        if (topic.Attributes.GetValue("LastModifiedBy", null) == null) {
+          topic.Attributes.SetValue("LastModifiedBy", options.CurrentUser);
+        }
+
       }
 
       /*------------------------------------------------------------------------------------------------------------------------