Promisify.all duplicate promisified functions on each call

[snovakovic]

By calling promisify.all on the same object multiple time we get multiple promisified versions of same function.

const test = { fn() => {} };
promisify.all(test); // creates fnAsync
promisify.all(test); // creates fnAsyncPromisified
promisify.all(test); // creates fnAsyncPromisifiedPromisified
etc...

Promisify.all should create promisified version of function just once no matter how many time it has been called.