title: Accordion has a default active panel, but it won't open
body: <p>I am using a frontend library called Element UI to create some accordions on my website. I got an array of objects that I like to create accordions for. FYI, from the element ui website: the <code>v-model</code> used on the accordion specifies the <code>currently active panel</code>. The <code>name</code> attribute is the unique identification of the panel. That means I can do: </p>
  
  <pre><code>&lt;el-collapse v-model="activeName" accordion&gt;
    &lt;el-collapse-item title="Consistency" name="1"&gt;
      &lt;div&gt;content content content&lt;/div&gt;
    &lt;/el-collapse-item&gt;
    &lt;el-collapse-item title="Consistency" name="2"&gt;
      &lt;div&gt;more content more content more content&lt;/div&gt;
    &lt;/el-collapse-item&gt;
  &lt;/el-collapse&gt;
  
  </code></pre>
  
  <p>One this loads, the first accordion will open since in the data object, <code>activeName</code> is set to <code>1</code>: </p>
  
  <pre><code>data() {
      return {
        activeName: '1',
      }
  }
  </code></pre>
  
  <p>Now I thought I'd just loop through the array and create accordions for the items in my array, and binding the <code>name</code> attribute to the <code>index + 1</code>, so that the first item will have the <code>name</code> attribute equal to <code>1</code>, the second to <code>2</code> etc. So:</p>
  
  <pre><code>&lt;el-collapse v-model="activeName" accordion&gt;
              &lt;el-collapse-item
                v-for="(item, index) in experience"
                :title="item.company"
                :name="index + 1"
                :key="index"
              &gt;
                &lt;div&gt; content content content &lt;/div&gt;
              &lt;/el-collapse-item&gt;
  &lt;/el-collapse&gt;
  </code></pre>
  
  <p>But for some reason, when the page loads, the first item in the accordion won't open automatically. They're all closed by default. I created a codesandbox with the problem that you can see for yourself here:  <a href="https://codesandbox.io/s/vue-template-no5eg" rel="nofollow noreferrer">codesandbox</a></p>