bulk_create throws NotNullViolation when DEFAULT constraint is present
I switched to using `default=datetime.today` with bulk_create for this use case, as it fills the field value before inserting. Peewee is not exactly aware of defaults implemented on the server. It can work with them, but it's not the same as using the `default=` python-side implementation.

To get bulk create w/a server-side default, use the `insert_many()` API instead:

```python
start_dates = [
    datetime.date(...),
    datetime.date(...),
    ...
]

Summary.insert_many([(date,) for date in start_dates], fields=[Summary.start_date]).execute()
```
I have the following model:
```
class Summary(Model):
    
    start_date = DateField(formats=['%Y-%m-%d'])
    end_date = DateField(formats=['%Y-%m-%d'], constraints=[SQL('DEFAULT CURRENT_DATE')])
    
    class Meta:
        database = db
        db_table = 'test_table'
```

If I insert using:
```
summary = Summary(start_date=date(2019,4,29))
summary.save()
```
it works. The debugging logs show:
```
2019-05-14 16:13:52 DEBUG  peewee:peewee:2928 - ('INSERT INTO "test_table" ("start_date") VALUES (%s) RETURNING "test_table"."id"', [datetime.date(2019, 4, 29)])
```

However if I try inserting using bulk_create:
```
Summary.bulk_create([summary], batch_size=100)
``` 
It throws:  
```
psycopg2.errors.NotNullViolation: null value in column "end_date" violates not-null constraint
```
The debugging logs show that the bulk_create query is different, which is causing the error.
```
2019-05-14 16:20:25 DEBUG  peewee:peewee:2928 - ('INSERT INTO "test_table" ("start_date", "end_date") VALUES (%s, %s) RETURNING "test_table"."id"', [datetime.date(2019, 4, 29), None])
```
I'm using PostgreSQL.