encode card into dictionary

def metainfo_to_dict(self) -> dict:
        '''encode card into dictionary'''

        r = {
            "version": self.version,
            "amount": self.amount,
            "number_of_decimals": self.number_of_decimals
        }

        if self.asset_specific_data:
            r.update({'asset_specific_data': self.asset_specific_data})

        return r