title: $store properties are not reactive when using computed property (Vuex)
body: <p>I have a Vuex store, which I'm injecting into my instance:</p>
  <pre class="lang-js prettyprint-override"><code>import store from '../store';
      
  const mainNav = new Vue({
    el: '#main-nav',
    store,
    components: { NavComponent }
  });
  </code></pre>
  <p>And I'm creating a computed property from that store in the component:</p>
  <pre class="lang-js prettyprint-override"><code>computed: {
    isWide() {
      return this.$store.state.nav.type === 'wide';
    }
  }
  </code></pre>
  <p>This does create the <code>this.isWide</code> property for the template on component initialization, but when the store value is updated, the component doesn't register this - the old value is still on the template.</p>
  <p>What am I doing wrong here?</p>