fix (k,v) in o bugs with falsy values and updates

src/directives/public/for.js

@@ -122,9 +122,9 @@ module.exports = {
  if (key) {
  frag.scope.$key = key
  }
- // update interator
+ // update iterator
  if (iterator) {
- frag.scope[iterator] = key || i
+ frag.scope[iterator] = key !== null ? key : i
  }
  // update data for track-by, object repeat &
  // primitive values.
@@ -218,7 +218,7 @@ module.exports = {
  _.define(scope, '$key', null)
  }
  if (this.iterator) {
- _.defineReactive(scope, this.iterator, key || index)
+ _.defineReactive(scope, this.iterator, key !== null ? key : index)
  }
  var frag = this.factory.create(host, scope, this._frag)
  frag.forId = this.id