ABI serialization not symmetric

[jnicoulaud-ledger]

Some unset values come back as explicitly set when saving/reloading:

        'abi': [
          {
            'name': 'approve',
            'inputs': [
              {
                'name': '_spender',
                'type': 'address',
              },
              {
                'name': '_value',
                'type': 'uint256',
              },
            ],
            'outputs': [
              {
                'name': '',
                'type': 'bool',
              },
            ],
            'stateMutability': 'nonpayable',
            'type': 'function',
+           'constant': False,
+           'payable': False,
          },

Must use bool | None = None to preserve difference between "set to default value" and "unset"