pg_dump losing index column collations for unique and primary keys

Hello,

I found that pg_dump fails to note down the collations used primary and
unique keys supporting indexes.
To reproduce:

create table f(a text);
create unique index f_pkey on f(a collate "C.UTF-8");
alter table f add primary key using index f_pkey;
\d f

pg_dump -t f

The only way to dump it correctly is to create indexes explicitly and
then use them in constraint definitions.
Please could someone have a look at the patch attached. Thank you.

Best,
  Alex