casting in ORDER BY

I try to fix foreign_data regression test for Czech locale (HEAD). For
example I replaced

SELECT * FROM information_schema.user_mapping_options ORDER BY 1, 2, 3,
4;

with following statement:

SELECT * FROM information_schema.user_mapping_options ORDER BY 1, 2,
3::name, 4;

which should use C sorting rules on thirds column, but it look likes
that third column is ignored. See diff:

(4 rows)

SET ROLE regress_test_role;
! SELECT * FROM information_schema.user_mapping_options ORDER BY 1, 2, 3, 4;
authorization_identifier | foreign_server_catalog | foreign_server_name | option_name | option_value
--------------------------+------------------------+---------------------+-------------+--------------
PUBLIC | regression | st1 | modified | 1
regress_test_role | regression | s5 | modified | 1
- regress_test_role | regression | s6 | username | test
regress_test_role | regression | st1 | password | boo
regress_test_role | regression | st1 | username | bob
(5 rows)

--- 711,723 ----
(4 rows)

SET ROLE regress_test_role;
! SELECT * FROM information_schema.user_mapping_options ORDER BY 1, 2, 3::name, 4;
authorization_identifier | foreign_server_catalog | foreign_server_name | option_name | option_value
--------------------------+------------------------+---------------------+-------------+--------------
PUBLIC | regression | st1 | modified | 1
regress_test_role | regression | s5 | modified | 1
regress_test_role | regression | st1 | password | boo
+ regress_test_role | regression | s6 | username | test
regress_test_role | regression | st1 | username | bob
(5 rows)

Is it bug or I overlook something?

Zdenek

PS: Regression test patch is attached.