Handle hierarchical choices in get_%s_display()

Bring the contributed method in line with Django's CharField.get_FOO_display() to handle named groups/categories

multiselectfield/db/fields.py

@@ -174,7 +174,7 @@ def contribute_to_class(self, cls, name):
         if self.choices:
             def get_list(obj):
                 fieldname = name
-                choicedict = dict(self.choices)
+                choicedict = dict(self.flatchoices)
                 display = []
                 if getattr(obj, fieldname):
                     for value in getattr(obj, fieldname):