Pass sort parameter into union in join function

Author: reidy-p

pandas/core/indexes/base.py

@@ -3256,7 +3256,7 @@ def join(self, other, how='left', level=None, return_indexers=False,
         elif how == 'inner':
             join_index = self.intersection(other, sort=sort)
         elif how == 'outer':
-            join_index = self.union(other)
+            join_index = self.union(other, sort=sort)
 
         if sort:
             join_index = join_index.sort_values()