fix serialization of subdocuments

In my application I have a schema like this:
```python
people = {
    'schema': {
         'name': {
             'type': 'string'
         },
         'information': {
             'type': 'dict',
             'schema': {
                 'best_friend': {
                     'type': 'objectid'
                  },
                 'birthday': {
                     'type': 'datetime'
                  }
             }
         }
    }
}
```
The 'information' subdocuments don't get serialized (converted to ObjectId/dateime Objects) resulting in the validation to fail:

```python
{
    "_issues": {
        "information": {
            "best_friend": "value '52f89dea4e03c0f71f64d334' cannot be converted to a ObjectId"
        }
    },
    "_status": "ERR"
}
```

This is because the subdocuments only get serialized if they define a schema themselves. I think the fields of a subdocument should get serialized the same way as first level fields.

regards
Hannes