Faster version

It's a faster version, that I use in my [classies lib](https://github.com/StephanHoyer/classies) that pretty much does the same thing.

Author: StephanHoyer

src/index.js

@@ -1,10 +1,9 @@
 export default function (obj) {
-	var k, cls='';
+	let cls
 	for (k in obj) {
 		if (obj[k]) {
-			cls && (cls += ' ');
-			cls += k;
+			cls = cls == null ? cls : cls + ' ' + k;
 		}
 	}
-	return cls;
+	return cls || '';
 }