Wrong checking of Promise in isPromise

[SmallhillCZ]

The function isPromise should not check if object is a Promise by Promise.resolve(object) === object. Instead it should check for .then() method.

https://github.com/editor-js/image/blob/e26b3e7c106486d2d776219e18cd125469991a25/src/uploader.js#L185-L187

This way custom uploader cannot be used when using for example patched Promises in ZoneJS.

Details explained here: https://stackoverflow.com/questions/27746304/how-do-i-tell-if-an-object-is-a-promise

[SmallhillCZ]

Moreover, I don't know about all the reasons of checking whether it is a Promise, but probably it is not needed at all. One should just use Promise.resolve(valueOrPromise) to get a Promise regardless of whether it was a value or a Promise before.

So if the only reason is to ensure the returned result is thenable, let me know, I will update the #136 to use Promise.resolve instead.