Refine README example (node-fetch#463)

This reverts commit fa6548e (node-fetch#441).

The autoClose option has been true by default since at least Node.js
v6.0.0. There is no need to set it once more.

Instead, make the example more realistic by handling stream outcomes
using a promise.

See node-fetch#375.

Author: TimothyGu

README.md

@@ -80,10 +80,19 @@ fetch('http://domain.invalid/')
 
 fetch('https://assets-cdn.github.com/images/modules/logos_page/Octocat.png')
 	.then(res => {
-		const dest = fs.createWriteStream('./octocat.png', {
-			autoClose: true,
+		return new Promise((resolve, reject) => {
+			const dest = fs.createWriteStream('./octocat.png');
+			res.body.pipe(dest);
+			res.body.on('error', err => {
+				reject(err);
+			});
+			dest.on('finish', () => {
+				resolve();
+			});
+			dest.on('error', err => {
+				reject(err);
+			});
 		});
-		res.body.pipe(dest);
 	});
 
 // buffer