Wrong checking of Promise in isPromise

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