Fix problem with calling the browser version concurrently

[stropitek]

• Uses closure on xhr to avoid referencing the wrong xhr object in the load callback if multiple requests are done in parallel
• Remove the abort method that was not exported so was useless

Closes #52

[sindresorhus]

Remove the abort method that was not exported so was useless

That's just a mistake. It should be exported. And it is documented to work like that. Can you properly export it (add it to those exported methods)?

[stropitek]

I was not sure what to do with the promise if the request gets canceled. In this implementation, the promise resolves with undefined as soon as the user calls cancel

[stropitek]

@sindresorhus anything I can do to get this landed?

I tested this implementation here. When cancelled, it aborts the current request and prevents the execution of the remaining fallback urls. And it resolves with undefined to avoid memory leaks (hanging promise that never resolves)

[sindresorhus]

And it resolves with undefined to avoid memory leaks (hanging promise that never resolves)

This violates the types and docs though as it's documented to always return a string if it doesn't error.

[sindresorhus]

The Node.js version throws a CancelError. I think the browser version should do the same.

https://github.com/sindresorhus/p-cancelable/blob/5e8b26853d994c1de14232ff839945fc331d2912/index.js#L3-L12

[stropitek]

Done! New sandbox: https://codesandbox.io/s/funny-payne-2qsmo?file=/src/index.js

[sindresorhus]

Always run $ npm test locally before pushing.

[sindresorhus]

Thank you :)