title: Element UI Table Column that uses slot-scope Issue "Cannot read property 'column_name' of undefined"
body: <p>Anyone here uses element ui framework? <a href="https://element.eleme.io/#/en-US/" rel="nofollow noreferrer">https://element.eleme.io/#/en-US/</a></p>
  
  <p>I'm using its table component and using slot-scope on its columns. It's working fine until I ran npm update which of course updates the packages. Now, console has a lot of errors. And later I discovered that this slot-scope of the table column causes the issue.</p>
  
  <p>Any help would be very much appreciated. Here is a fiddle of the issue.</p>
  
  <p><a href="https://jsfiddle.net/japhfortin/jkzma0v8/3/" rel="nofollow noreferrer">https://jsfiddle.net/japhfortin/jkzma0v8/3/</a></p>
  
  <pre><code>  &lt;el-table :data=list&gt;
      &lt;el-table-column label="First Name"&gt;
        &lt;template slot-scope="scope"&gt;{{ scope.row.first_name }}&lt;/template&gt;
      &lt;/el-table-column&gt;
      &lt;el-table-column prop="last_name" label="Last Name"&gt;
      &lt;/el-table-column&gt;
    &lt;/el-table&gt;
  
    data() {
      return {
        input: '',
        list: [
        {
          first_name: 'Foo',
          last_name: 'Bar'
        }
        ]
      }
    },
  </code></pre>