Can not use PickledField with database proxy
Fixed in b2d52ba7973e6d980ed9efae36322c16e9fec074 see #933 

The following code raise an error

``` python
from peewee import *
from playhouse.fields import *

database_proxy = Proxy()

db = SqliteDatabase('test.db')

class BaseModel(Model):
    class Meta:
        database = database_proxy


class TestModel(BaseModel):
    data = PickledField()


database_proxy.initialize(db)

db.create_tables([TestModel])
```

the error:
`raise AttributeError('Cannot use uninitialized Proxy.')`
