Treat subclasses with interface types the same as parent classes with interface types

- All tests passing

Author: tgmayfield

src/FluentNHibernate/Visitors/SeparateSubclassVisitor.cs

@@ -91,7 +91,7 @@ private IDictionary<int, IList<IIndeterminateSubclassMappingProvider>> SortByDis
                 var subclassType = subclassProvider.EntityType;
                 var level = 0;
 
-                bool implOfParent = parentType.IsInterface
+                bool implOfParent = (parentType.IsInterface || subclassType.IsInterface)
                     ? DistanceFromParentInterface(parentType, subclassType, ref level)
                     : DistanceFromParentBase(parentType, subclassType.BaseType, ref level);