title: vuex computed properties are not reactive
body: <p>test.vue, the 'test' property is reactive in vue component, but 'title' property from vuex store is not reactive when I changing the value of the first input tag. I found some similar questions but the solutions are not work for me. where did I do wrong here? </p>
  
  <p><strong>template</strong></p>
  
  <pre><code>&lt;div class="title"&gt;
    &lt;input 
      type="text" 
      placeholder="palceholder" 
      v-model="title"
     /&gt;
  &lt;/div&gt;
  &lt;input v-model="test"/&gt;
  &lt;div&gt;{{test}}&lt;/div&gt;
  &lt;div&gt;
    {{title}}
  &lt;/div&gt;
  </code></pre>
  
  <p><strong>js</strong></p>
  
  <pre><code>data() {
    test: 'test'
  },
  computed: {
    ...mapGetters(['article']),
    title: {
      get () { 
         return this.article.title || '' 
      },
      set (value) { this.$store.commit(Types.UPDATE_TITLE, value) }
    }
  }
  </code></pre>
  
  <p><strong>store.js</strong></p>
  
  <pre><code>import * as Types from '../mutation-types'
  import Vue from 'vue'
  const state = {
    article: {}
  }
  
  const getters = {
    article: state =&gt; state.article
  }
  
  const mutations = {
    [Types.UPDATE_TITLE] (state, title) {
      state.article = { ...state.article, title }
    }
  }
  </code></pre>