Pass desired array type from pg.array_agg to functions.array_agg

Pass desired array type from pg.array_agg to functions.array_agg

Fixed the :func:`.postgresql.array_agg` function, which is a slightly
altered version of the usual :func:`.functions.array_agg` function, to also
accept an incoming "type" argument without forcing an ARRAY around it,
essentially the same thing that was fixed for the generic function in 1.1
in :ticket:`4107`.

Fixes: #4324
Change-Id: I399a29f59c945a217cdd22c65ff0325edea8ea65