Fix logic error
Thanks, I've committed equivalent patch with a testcase. I appreciate this fix!!

## Fixed
- fix logic error in `playhouse.shortcuts.model_to_dict`
  if `backrefs = True`,  only `descriptor` and `foreign_key` fields both not in the `only` will excute `continue`

```
class User(Model):
    name = CharField()

class Tweet(Model):
    use = ForeignKeyField(User, related_name='tweets')

user = User(name='tim')
tweet = Tweet(user=user, content='my first tweets')
```

I  define two model  `User` and `Tweet`, and when I use `model_to_dict`, the output is

```
d = model_to_dict(user, only=[User.name, User.tweets, Tweet.content], backrefs=True)
print(d)
>>> {'name': 'tim'}
```

but the expect results should be

```
>>> {'name': 'tim', 'tweets': {'content': 'my first tweets'}}
```

This PR fixed this issue
