Always use a fresh iterator

[sebald]

In 2.0 there was a regression introduced regarding the reusability of the pipe.

const pipe = require('p-pipe');

const fn = () => 'whoops';
const task = pipe(fn);

const t1 = await task(); // returns 'whoops'
const t2 = await task(); // returns undefined

This was caused by not resetting the iterator. Every subsequent use of the task was reusing the same iterator, which already finished iterating in the first usage 🤦‍♂️

This patch will make sure a fresh iterator is created on every usage of the task.

Also added a test to make sure this is not happening again!

Fixes #6

[johnmaia]

@sindresorhus would appreciate if you could take a look at this PR and release a 🌈 new version 🦄.