fix(reactivity): trigger Map custom has correctly

[Picknight]

Fix the following situation:

let map = new Map()
map.has = function(key) {
  console.log('trigger has')
  return Map.prototype.has.call(map, key)
}

let proxy = reactive(map)
proxy.has('k')  // print 'trigger has' twice

btw, I am not sure if the following situation is as expected: (this PR did not affect this)

let map = new WeakMap()
map.has = function(key) {
  console.log('trigger has')
  return WeakMap.prototype.has.call(map, key)
}

let proxy = reactive(map)
proxy.has(reactive({}))  // print 'trigger has' twice

[yyx990803]

Calling twice on reactive keys is necessary for the desired behavior. Wouldn't really say it's a bug.