Return a json dictionary representing this model.

def _to_dict(self):
        """Return a json dictionary representing this model."""
        _dict = {}
        if hasattr(self, 'features') and self.features is not None:
            _dict['features'] = self.features
        if hasattr(self,
                   'text_characters') and self.text_characters is not None:
            _dict['text_characters'] = self.text_characters
        if hasattr(self, 'text_units') and self.text_units is not None:
            _dict['text_units'] = self.text_units
        return _dict