FIXED: Error when binding several objects to the same objectController

Error when binding several objects to the same objectController.selection.keyPath and objectController selection changes. Fixes issue #2039

This is basically similar to a fix in cocotron (from where the code is borrowed) but rewritten with modern code.
Cocotron :  https://code.google.com/p/cocotron/source/browse/AppKit/NSController/NSControllerSelectionProxy.m