FIX Handle case where stop is None

Author: jnothman

sklearn/feature_extraction/text.py

@@ -88,6 +88,8 @@ def _check_stop_list(stop):
         return ENGLISH_STOP_WORDS
     elif isinstance(stop, six.string_types):
         raise ValueError("not a built-in stop list: %s" % stop)
+    elif stop is None:
+        return None
     else:               # assume it's a collection
         return frozenset(stop)