Update the value for a field(s) in the listitem

        :param update: A dict of {'field name': newvalue}

def update_fields(self, updates):
        """
        Update the value for a field(s) in the listitem

        :param update: A dict of {'field name': newvalue}
        """

        for field in updates:
            if self._valid_field(field):
                self._track_changes.add(field)
            else:
                raise ValueError('"{}" is not a valid internal field name'.format(field))

        # Update existing instance of fields, or create a fields instance if needed
        if self.fields:
            self.fields.update(updates)
        else:
            self.fields = updates