[PATCH] COPY TO FORMAT json: respect column list order

Hi,

While testing the new COPY JSON format, I noticed the column list
ordering is ignored when the list mentions every column of the
relation. CSV and TEXT formats respect the order; JSON does not.

CREATE TABLE t (a int, b int);
INSERT INTO t VALUES (1, 2);

COPY t (b, a) TO STDOUT WITH (FORMAT csv); -- "2,1"
COPY t (b, a) TO STDOUT WITH (FORMAT json); -- {"a":1,"b":2}
-- expected: {"b":2,"a":1}

In BeginCopyTo() (commit 7dadd38cda9), the JSON path builds a custom
TupleDesc only when the column list is shorter than the relation:

if (rel && list_length(cstate->attnumlist) < tupDesc->natts)

When the user provides a full column list reordered, list_length
equals natts, so the natural TupleDesc is reused.

Fix is to extend the condition to also fire when an explicit column list
was supplied (attnamelist != NIL).

All 245 regression tests pass.

Thanks,
Baji Shaik.