Function promisify

tests/unit/api/objectGetAttributes.js:26–34

// Promisify helper for functions with non-standard callback signatures
const promisify = fn => (...args) => new Promise((resolve, reject) => {
    fn(...args, (err, ...results) => {
        if (err) {
            reject(err);
        } else {
            resolve(results);
        }
    });
});

const bucketPutAsync = promisify(bucketPut);
const bucketPutVersioningAsync = promisify(bucketPutVersioning);