Bug Fix: Ensure ClearItems() isn't modifying the enumerated collection

The `ClearItems()` method was throwing an exception because it was attempting to modify the `Items` collection while iterating over it (abfe213). Easily fixed with a `ToList()`.

Author: JeremyCaney

OnTopic/Collections/Specialized/TrackedRecordCollection{TItem,TValue,TAttribute}.cs

@@ -632,7 +632,7 @@ protected override void RemoveItem(int index) {
     ///   attributes to be handled individually.
     /// </remarks>
     protected override void ClearItems() {
-      foreach (var item in Items) {
+      foreach (var item in Items.ToList()) {
         Remove(item);
       }
       base.ClearItems();