title: How to correctly use Nuxt / Vue ...mapMutations
body: <p>I'm trying to use <code>...mapMutations</code> with Vuex modules in Nuxt. My call to <code>this.setDates(dates)</code> results in an error:</p>
  
  <blockquote>
    <p>this.setDates is not a function</p>
  </blockquote>
  
  <p><strong>In my Nuxt store: <code>store/header.js</code></strong></p>
  
  <pre><code>export const mutations = {
      setDates(state, dates) {
          state.dates = dates;
      },
  }
  </code></pre>
  
  <p><strong>In my component</strong></p>
  
  <pre><code>methods: {
      ...mapMutations(
          {'header': ['setDates']},
      ),
      changeDate(dates) {
          this.setDates(dates);
      }
  }
  </code></pre>