[IMP] vdom: performance improvement

Port from original snabbdom project: snabbdom/snabbdom#634

The issue is that before this commit, the removeClass method was
sometimes called even if it is not useful. See this comment for more
detail: https://github.com/snabbdom/snabbdom/issues/633#issue-618706258

src/vdom/modules.ts

@@ -231,7 +231,8 @@ function updateClass(oldVnode: VNode, vnode: VNode): void {
   elm = vnode.elm as Element;
 
   for (name in oldClass) {
-    if (name && !klass[name]) {
+    if (name && !klass[name] && !Object.prototype.hasOwnProperty.call(klass, name)) {
+      // was `true` and now not provided
       elm.classList.remove(name);
     }
   }