Unexpected wrong inserts into the user type

```
First show that in the pure PostgreSQL all fine.
=> create type b as (x int,y int);
CREATE TYPE
=> create table a (a int,b b);
CREATE TABLE
=> insert into a (a,b.x,b.y) values (0,0,0);
INSERT 0 1
=> select * from a;
a | b
---+-------
0 | (0,0)
(1 row)

Now create type on the work nodes.
=> \c - - - 9701
*****
=> create type b as (x int,y int);
CREATE TYPE
=> \c - - - 9702
*****
=> create type b as (x int,y int);
CREATE TYPE

Return to the coordinator and distrib table
=> \c - - - 9700
*****
=> select create_distributed_table('a','a');
NOTICE: Copying data from local table...
create_distributed_table
--------------------------

(1 row)
=> insert into a (a,b.x,b.y) values (1,1,1);
INSERT 0 1
=> select * from a;
a | b
---+-------
0 | (0,0)
1 | (1,)
(2 rows)
```
As you can see problem is only on inserting in the distributed table. And easy reapitable.
Ubuntu 17.10
$ dpkg -s postgresql-10|grep Version
Version: 10.1-1.pgdg17.04+1
$ dpkg -s postgresql-10-citus-7.3|grep Version
Version: 7.3.0.citus-1

Start by reproducing the SQL sequence in the issue with PostgreSQL 10.1 and Citus 7.3.0, comparing the coordinator and worker type definitions and the distributed-table insert. Done means inserting into the distributed table preserves both fields of the composite value, matching the local PostgreSQL result.

Written by the indexing model from the issue text.