Merge pull request #3512 from jakevdp/nbconvert_filter

fix nbconvert filter validation

IPython/nbconvert/exporters/exporter.py

@@ -160,10 +160,7 b' class Exporter(Configurable):'
         #Load user filters.  Overwrite existing filters if need be.
         if not filters is None:
             for key, user_filter in filters.iteritems():
-                if issubclass(user_filter, MetaHasTraits):
-                    self.environment.filters[key] = user_filter(config=config)
-                else:
-                    self.environment.filters[key] = user_filter
+                self.register_filter(key, user_filter)
 
     @property
     def default_config(self):