Update variable description on upsert

[codenirvana]

const list = new VariableList(null, [{
    key: 'k1',
    value: 'initial',
    description: 'initial description'
}]);

list.upsert({
    key: 'k1',
    value: 'updated',
    description: 'updated description'
});

list.toJSON();
// [{
//     description: {
//         content: 'initial description',
//         type: 'text/plain'
//     },
//     type: 'any',
//     value: 'updated',
//     key: 'k1'
// }]

[csk1827]

@codenirvana I could find this bug still reproducible. Can I take up this one?

[codenirvana]

@csk1827 Sure 👍

[codenirvana]

Fixed by #1332