Adding na_position for multiindex

Author: brandonmburroughs

pandas/core/series.py

@@ -1782,7 +1782,8 @@ def sort_index(self, axis=0, level=None, ascending=True, inplace=False,
                                                  sort_remaining=sort_remaining)
         elif isinstance(index, MultiIndex):
             from pandas.core.groupby import _lexsort_indexer
-            indexer = _lexsort_indexer(index.labels, orders=ascending)
+            indexer = _lexsort_indexer(index.labels, orders=ascending,
+                                       na_position=na_position)
 
         else:
             from pandas.core.groupby import _nargsort