Get an iterable with GCS folder contents.
        Iterable contains paths relative to queried path.

def listdir(self, path):
        """
        Get an iterable with GCS folder contents.
        Iterable contains paths relative to queried path.
        """
        bucket, obj = self._path_to_bucket_and_key(path)

        obj_prefix = self._add_path_delimiter(obj)
        if self._is_root(obj_prefix):
            obj_prefix = ''

        obj_prefix_len = len(obj_prefix)
        for it in self._list_iter(bucket, obj_prefix):
            yield self._add_path_delimiter(path) + it['name'][obj_prefix_len:]