fix(watch): align behavior with vue-next(doWatch).

[ygj6]

This PR can solve the problem of not triggering watch callbacks in the following scenarios，keep the same behavior with vue-next.

<template>
  <div id="app">
    <div>{{ count[0] }}</div>
    <button @click="change">jml</button>
  </div>
</template>

<script>
import {watch,set, reactive} from '@vue/composition-api'

export default {
  name: "App",  
  setup() {
    const count = reactive(["sdfs"])
    const change = ()=>{
      set(count, 0, "222")
    }
    watch(count,()=>{console.log("xxxxx");},{deep: true}
    )
    return {count,change};
  },
};
</script>

<style>
#app {
  font-family: Avenir, Helvetica, Arial, sans-serif;
  -webkit-font-smoothing: antialiased;
  -moz-osx-font-smoothing: grayscale;
  text-align: center;
  color: #2c3e50;
  margin-top: 60px;
}
</style>