Remove an undefined attribute instead of setting it to "undefined" (string)

src/shared/dom.js

@@ -82,7 +82,8 @@ export function removeListener(node, event, handler) {
 }
 
 export function setAttribute(node, attribute, value) {
-	node.setAttribute(attribute, value);
+	if (value === undefined) removeAttribute(node, attribute);
+	else node.setAttribute(attribute, value);
 }
 
 export function setAttributes(node, attributes) {
@@ -90,8 +91,7 @@ export function setAttributes(node, attributes) {
 		if (key in node) {
 			node[key] = attributes[key];
 		} else {
-			if (attributes[key] === undefined) removeAttribute(node, key);
+			setAttribute(node, key, attributes[key]);
-			else setAttribute(node, key, attributes[key]);
 		}
 	}
 }

test/runtime/samples/set-undefined-attr/_config.js

@@ -0,0 +1,5 @@
+export default {
+	'skip-ssr': true,
+
+	html: '<div></div>'
+};

test/runtime/samples/set-undefined-attr/main.html

@@ -0,0 +1,14 @@
+<div {foo} {bar}></div>
+
+<script>
+	export default {
+		data: () => ({
+			foo: 1,
+			bar: undefined
+		}),
+
+		oncreate () {
+			this.set({ foo: undefined });
+		}
+	};
+</script>