always perform requested trait assignments

Some assignments do not change *value*, but do change the object,
and these should not be ignored.

For instance, container objects are currently impossible to assign
if they evaluate as equal.

IPython/utils/traitlets.py

@@ -302,8 +302,8 b' class TraitType(object):'
     def __set__(self, obj, value):
         new_value = self._validate(obj, value)
         old_value = self.__get__(obj)
+        obj._trait_values[self.name] = new_value
         if old_value != new_value:
-            obj._trait_values[self.name] = new_value
             obj._notify_trait(self.name, old_value, new_value)
 
     def _validate(self, obj, value):