Implicit add Primary Key for id field
This is just the standard peewee behavior. If you want a different field to be primary key, declare a different one.

```
class Model(BaseModel):
    id = TextField()
    field = TextField()
```

Field id implicitly is primary key now. TextField, for example, is not really suitable to be a primary key, but unexpectedly it is. Maybe this situation need report warning or even error.
