BUG #14759: insert into foreign data partitions fail

The following bug has been logged on the website:

Bug reference: 14759
Logged by: Murat Tuncer
Email address: mtuncer(at)gmail(dot)com
PostgreSQL version: 10beta2
Operating system: Mac 10.12.6
Description:

I got

ERROR: cannot route inserted tuples to a foreign table

when trying to insert into a partitioned table when the target is a foreign
table.

First observed the error when trying to have cstore_fdw tables as partitions
of a regular table. Reproduced the problem with postgres_fdw too.

Details
- create table ptest(a int, b int) partition by range (a);
- create table pt1 partition of ptest for values from (1) to (10);
- create foreign table pt2 partition of ptest for values from (11) to (20)
server foreign_server;

- insert into ptest values (1, 2) ---> works
- insert into ptest values (11, 12) --> fails with mentioned error
message.

I had successfully reproduced this for cstore_fdw and postgres_fdw servers.
Insert works fine if I try to insert into partition table directly.