message: fix(core): only unset dom prop when not present
  fix
diff: @@ -19,10 +19,11 @@ function updateDOMProps (oldVnode: VNodeWithData, vnode: VNodeWithData) {
  }
  for (key in oldProps) {
  - if (isUndef(props[key])) {
  + if (!(key in props)) {
  elm[key] = ''
  }
  }
  +
  for (key in props) {
  cur = props[key]
  // ignore children if the node has textContent or innerHTML,
  