domain on bit(N) type produces strange results

Creating a domain on bit(N) doesn't seem to work as expected when casting
to the domain type.

CREATE DOMAIN bit4 AS bit(4);

SELECT 7::bit(4), 7::bit4;
bit | bit4
------+------
0111 | 1000
(1 row)

Reported in #postgresql by msw_alt.

Kris Jurka