Fix editing on forms.

taggit/managers.py

@@ -72,6 +72,9 @@ class TaggableManager(object):
         defaults.update(kwargs)
         return form_class(**kwargs)
     
+    def value_from_instance(self, instance):
+        return getattr(instance, self.name).all()
+    
     def related_query_name(self):
         return None