Examples in README.md use XmlStream unnecessarily

[davetapley]

The docs show XmlStream being used to build a filesArray:

s3-zip/README.md

Lines 92 to 101 in 1c21f82

	const xml = new XmlStream(files)
	xml.collect('Key')
	xml.on('endElement: Key', function(item) {
	filesArray.push(item['$text'].substr(folder.length))
	})
	xml
	.on('end', function () {
	zip(filesArray)
	})

---

But aws-sdk returns JS objects automatically, e.g:

  const {Contents} = await s3.listObjects(params).promise()
  const keys = Contents.map(({Key}) => Key

So why do we need XmlStream?

[orangewise]

I’ve not tested this part myself. Maybe you can add a test and update the readme? I’m happy to merge a PR.