ENH O(1) stop-word lookup when list provided

The docstring says stop_words can be a list, but it should be accessed as a set.

Author: jnothman

sklearn/feature_extraction/text.py

@@ -89,7 +89,7 @@ def _check_stop_list(stop):
     elif isinstance(stop, six.string_types):
         raise ValueError("not a built-in stop list: %s" % stop)
     else:               # assume it's a collection
-        return stop
+        return frozenset(stop)
 
 
 class VectorizerMixin(object):