Clean index values after loading.

def _clean_index(self):
        "Clean index values after loading."
        for idx_name, idx_def in self.index_defs.items():
            if idx_def['type'] == 'lazy':
                self.build_index(idx_name)
        for index_name, values in self.indexes.items():
            for value in values:
                if not isinstance(values[value], set):
                    values[value] = set(values[value])