Implement objectsAtIndexes on _CPObservableArray to return another _CPObservableArray instance

AppKit/CPObjectController.j

@@ -455,6 +455,11 @@ var CPObjectControllerObjectClassNameKey                = @"CPObjectControllerOb
     [super removeObjectAtIndex:anIndex];
 }
 
+- (_CPObservableArray)objectsAtIndexes:(CPIndexSet)theIndexes
+{
+    return [_CPObservableArray arrayWithArray:[super objectsAtIndexes:theIndexes]];
+}
+
 - (void)addObject:(id)anObject
 {
    [self insertObject:anObject atIndex:[self count]];