BUG #15623: Inconsistent use of default for updatable view

The following bug has been logged on the website:

Bug reference:      15623
Logged by:          Roger Curley
Email address:      rocurley@gmail.com
PostgreSQL version: 11.1
Operating system:   Ubuntu 11.1
Description:

Steps to reproduce (run in psql shell):
```
DROP TABLE IF EXISTS test CASCADE;
CREATE TABLE test (
  id int PRIMARY KEY,
  value int DEFAULT 0
);
CREATE VIEW test_view AS (SELECT * FROM test);

INSERT INTO test_view VALUES (1, DEFAULT), (2, DEFAULT);
INSERT INTO test VALUES (3, DEFAULT), (4, DEFAULT);
INSERT INTO test_view VALUES (5, DEFAULT);
SELECT * FROM test;
```

Result:
```
 id | value 
----+-------
  1 |      
  2 |      
  3 |     0
  4 |     0
  5 |     0
```

Expected Result:
```
 id | value 
----+-------
  1 |     0
  2 |     0
  3 |     0
  4 |     0
  5 |     0
```
In particular, it's surprising that inserting 1 row into an updatable view
uses the table default, while inserting 2 uses null.