shortcuts.model_to_dict convert 0 to None in ForeignKeyField unless recurse == False
Thanks!
```python
import peewee
from playhouse.shortcuts import model_to_dict
database = peewee.PostgresqlDatabase(
    'temp_db',
   ....
    )
database.connect()

class Group (peewee.Model):
    id = peewee.BigIntegerField(null= False, index = True, unique = True, primary_key = True)
    class Meta():
        database = database
class User (peewee.Model):
    id = peewee.BigIntegerField(null= False, index = True, unique = True, primary_key = True)
    group_id = peewee.ForeignKeyField(Group, null = False)

    class Meta():
        database = database

database.create_tables([Group, User])
Group.create(id = 0)
User.create(id = 0, group_id = 0)
print (model_to_dict(User.get_by_id(0))['group_id'] == 0) # False
print (model_to_dict(User.get_by_id(0), False)['group_id'] == 0) #True
```
