{AttributeError}'ModelOptions' object has no attribute 'declared_fields'
Issue with versions in my systems library and within the project.

Attempting a basic command, but it appears that "declared_fields" in the _meta is not a valid field.
See line in code [here](https://github.com/coleifer/peewee/blob/master/playhouse/shortcuts.py#L107). 

**Model:**

```
db = AppEngineDatabase('adb')

class CloudSQLModel(Model):
    create_date = DateField(default=datetime.datetime.now)
    last_modified = DateField(default=datetime.datetime.now)
    modified_by = CharField(null=True)  #ndb key of the user that modified it

    class Meta:
        database = db  # This model uses the "agent.db" database


# Add the initial items for insertion into our datastore
class SourcedModel(CloudSQLModel):
    # Create the available status of the property
    STATE_CHOICES = (
        (0, 'new'),
        (1, 'csg_cleaned'),
        (2, 'third_party_cleaned'),
        (3, 'ready'),
        (4, 'released'),
        (5, 'expired'),
        (9, 'deleted'))

    # Source of the location that we found... usually a URL will suffice
    source = CharField(null=True)

    # Date of the item last touched upon
    last_seen_date = DateField(null=True)

    # Status of the property... we do not delete, just disable
    state = IntegerField(default=0, choices=STATE_CHOICES)

    # Return the status of the item
    def get_status_label(self):
        return dict(self.STATUS_CHOICES)[self.status]

class NationalProducerNumber(SourcedModel):
    number = CharField()
```

**Create new item and attempt to print model.**

```
npn = NationalProducerNumber.create(
                    number="001193381901"
                )
print(model_to_dict(non))
```

When I run the code shown, I receive the following Attribute Error:
`'ModelOptions' object has no attribute 'declared_fields`

Declared fields does not seem to be an attribute of the _meta object. `fields` is present, but `declared_fields` is not. 

Am I missing a key step in this?
