bug in "create unique index"

when creating a index unique in a table that accept nulls the unique
constraint doesn't work. 

Example:
---
create table test_unique(i1 integer, i2 integer, unique(i1,i2);
insert into test_unique(1,null);
insert into test_unique(1,null);
insert into test_unique(1,null);
---
all "inserts" above insert sucefully.