COPY TO FORMAT JSON: respect column list order

COPY TO FORMAT JSON: respect column list order

When COPY TO with FORMAT json is given an explicit column list that
names all columns in a different order, the JSON output incorrectly
used the table's physical column order instead of the requested order.

This happened because BeginCopyTo() only built a restricted TupleDesc
when list_length(attnumlist) < tupDesc->natts.  When all columns are
listed (just reordered), this condition was false and no projected
TupleDesc was built, causing CopyToJsonOneRow() to emit columns in
physical order.

Fix by also building the projected TupleDesc when an explicit column
list was provided (attnamelist != NIL), even if it names all columns.

Author: Baji Shaik <baji.pgdev@gmail.com>
Reviewed-by: Andrew Dunstan <andrew@dunslane.net>
Discussion: https://postgr.es/m/CA+fm-ROd4cNKM524n6EdgtZ9xOzOHJDNv8J_9Mvr2+2t1qWSDw@mail.gmail.com