Fix reset magic not properly clearing {_, _, ___}

IPython/core/displayhook.py

@@ -298,7 +298,7 b' class DisplayHook(Configurable):'
         self._, self.__, self.___ = '', '', ''
 
         if '_' not in builtin_mod.__dict__:
-            self.shell.user_ns.update({'_':None,'__':None, '___':None})
+            self.shell.user_ns.update({'_':self._,'__':self.__,'___':self.___})
         import gc
         # TODO: Is this really needed?
         # IronPython blocks here forever