[FIX] component: reset isRendered when reusing fiber

It is important to reset the isRendered flag to false to ensure
that other (subsequent) simultaneous calls to render will be
skipped, as the currentFiber is actually no yet (re-)rendered.

Closes #483

src/component/fiber.ts

@@ -98,6 +98,7 @@ export class Fiber {
   _reuseFiber(oldFiber: Fiber) {
     oldFiber.cancel(); // cancel children fibers
     oldFiber.isCompleted = false; // keep the root fiber alive
+    oldFiber.isRendered = false; // the fiber has to be re-rendered
     if (oldFiber.child) {
       // remove relation to children
       oldFiber.child.parent = null;