Return a list of all the fields that should be lowercased

        This is done on fields with `lower=True`.

def to_lower(cls):  # NOQA
        """ Return a list of all the fields that should be lowercased

        This is done on fields with `lower=True`.
        """

        email = cls.get_fields_by_class(EmailType)
        lower = cls.get_fields_by_prop('lower', True) + email

        return list(set(email + lower))