Fix up JOIN .. USING with domains

The attached fixes select_common_type() to support the below case:

create table t1( c1 int);
create domain dom_c1 int;
create table t2(c1 dom_c1);
select * from t1 join t2 using( c1 );

I didn't see a need for maintaining the domain as the preferred type. A
simple getBaseType() call on all elements of the list seems to be
enough.

--
Rod Taylor <rbt(at)rbt(dot)ca>

PGP Key: http://www.rbt.ca/rbtpub.asc