Make .iterator() implement return() correctly

[raymondfeng]

Please note the current implementation of return() breaks the AsyncIterator interface. It should return a promise.

interface AsyncIterator<T> {
    next(value?: any): Promise<IteratorResult<T>>;
    return?(value?: any): Promise<IteratorResult<T>>;
    throw?(e?: any): Promise<IteratorResult<T>>;
}

[raymondfeng]

@sindresorhus @szmarczak One more thing to review. Thanks!

[raymondfeng]

@sindresorhus Thank you for the prompt merge. Would you please publish a new release soon?

[sindresorhus]

Yes, when #16 is done.