Ensure that RemoveItem() enforces business logic

Previously, when deleting items directly via `Remove()`, the `RemoveItem()` wouldn't enforce business logic. This is problematic because sometimes properties either a) disallow nulls, or b) must update their internal states when values are set to null. This would be handled when calling `SetValue()` or going directly through the property, but bypassed when calling `Remove()`.

This update patches that hole, and thus resolves the `Remove()` portion of #79.

Author: JeremyCaney

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

@@ -606,11 +606,15 @@ protected override void SetItem(int index, TItem item) {
     ///   cref="TrackedRecord{T}"/>s are marked as <see cref="TrackedRecord{T}.IsDirty"/>.
     /// </remarks>
     protected override void RemoveItem(int index) {
-      var trackedRecord = this[index];
-      if (!AssociatedTopic.IsNew) {
-        DeletedItems.Add(trackedRecord.Key);
+      var trackedRecord = this[index] with {
+        Value = null
+      };
+      if (_topicPropertyDispatcher.Enforce(trackedRecord.Key, trackedRecord)) {
+        if (!AssociatedTopic.IsNew) {
+          DeletedItems.Add(trackedRecord.Key);
+        }
+        base.RemoveItem(index);
       }
-      base.RemoveItem(index);
     }
 
     /*==========================================================================================================================