Cannot pass callable as `slice_source`

[forman]

Calling zappend(slice_paths, config=config, slice_source=MySliceSource) where MySliceSource is a class, raises a ValidationError. This actually ok since validation is done against a JSON schema. However, it should be allowed to pass a class or function in addition to its fully qualified object name.

The error:

ValidationError: <class '__main__.MySliceSource'> is not of type 'string'

Failed validating 'type' in schema['properties']['slice_source']:
    {'description': 'The fully qualified name of a class or function that '
                    'provides a slice source for each slice item. If a '
                    'class is given, it must be  derived from '
                    '`zappend.api.SliceSource`. If a function is given, it '
                    'must return an instance of  '
                    '`zappend.api.SliceSource`. Refer to the user guide '
                    'for more information.',
     'minLength': 1,
     'type': 'string'}

On instance['slice_source']:
    <class '__main__.MySliceSource'>