SqliteDatabase + DateTimeField not working
Too much typing...
```Python
expires: DateTimeField(default=datetime.now(timezone.utc))
```
Should be:

```Python
expires: DateTimeField = DateTimeField(default=datetime.now(timezone.utc))
```
When you run the following example, it creates a DB and the `wnstoken` table but without the `expires` field.
```python
from peewee import *
from datetime import datetime, timezone

DB_PATH: str = "peeweeTest.db"
db: SqliteDatabase = SqliteDatabase(DB_PATH)

class WNSToken(Model):
    token: CharField = CharField(unique=True)
    expires: DateTimeField(default=datetime.now(timezone.utc))

    class Meta:
        database = db

print("Initalizing the DB...")
db.connect()
db.create_tables([WNSToken])
print("DB initialized.")
```

It looks like it's ignoring `DateTimeField`s. I tested this with multiple different combinations of fields and tables. As soon as a table includes a `DateTimeField`, it won't be created.

Inserting works, but also completely ignores the `DateTimeField`.

### Environment
* peewee: `3.13.2`
* Python3: `3.7.6`

### Resulting DB
![Screenshot from 2020-04-01 14-21-18](https://user-images.githubusercontent.com/11741404/78136556-206ed300-7424-11ea-843a-5fd65116b69f.png)
