test_client.py: metadata deserialization is not working

`test_level()` works and `test_get_submodel_metadata` does not, even though internally in the server both do the same thing. The problem with the metadata deserialization seems to be that the client always thinks that the model_type is equall to None even though it isn't. 
You always get:
```
ValueError: Invalid value for `model_type`, must not be `None`
```