generate correct plan for INSERT .. SELECT .. FROM generate_series()

generate correct plan for INSERT .. SELECT .. FROM generate_series()

The planner now generates the correct plan

                                             QUERY PLAN
    ---------------------------------------------------------------------------------------------
     Remote Subquery Scan on all (dn1,dn2)  (cost=100.00..119.00 rows=1000 width=12)
       ->  Insert on t1  (cost=100.00..119.00 rows=1000 width=12)
             ->  Remote Subquery Scan on all (dn2)  (cost=100.00..119.00 rows=1000 width=12)
                   Distribute results by H: i
                   ->  Function Scan on generate_series s  (cost=0.00..10.00 rows=1000 width=12)
    (5 rows)

for queries like

    INSERT INTO t1 SELECT i, i, i FROM generate_series(1,1000) s(i);

Sadly no rows are inserted, for some reason. But the plan looks fine
(at least it seems similar to 9.5).