title: sortable issue in element-ui
body: <p>I have wrote the table code by using Element-ui. I need the column sortable, but however it does not work. I have no idea what is the reason. Is it the data format wrong?</p>
  
  <p>great thx</p>
  
  <pre><code>&lt;el-table v-loading="listLoading" :data="list"&gt;
    &lt;el-table-column type="selection" &gt;&amp;nbsp;&lt;/el-table-column&gt;
    &lt;el-table-column label="NO" fixed width="100" sortable&gt;&lt;template slot-scope="scope"&gt;{{ scope.row.number }}&lt;/template&gt;&lt;/el-table-column&gt;
    &lt;el-table-column v-for="head in formThead" :key="head.value" :label="head.title" :min-width="head.celwidth ==null ? 100: head.celwidth" :sortable="head.sortable"&gt;
      &lt;template slot-scope="scope"&gt;
        {{ scope.row[head.value] }}
      &lt;/template&gt;
    &lt;/el-table-column&gt;
  &lt;/el-table&gt;
  
  
  formThead: [
      { value: 'dest', title: 'Dest', celwidth: 110, sortable: true }, { value: 'date', title: 'Date' }, { value: 'pcs', title: 'PCS', celwidth: 80 }],
  
  list：[
    {"number": "2", "dest": "AAA", "date": "2019-01-01", "pcs": "2"},
    {"number": "3", "dest": "ABB", "date": "2019-01-02", "pcs": "1"},
    {"number": "4", "dest": "CAA", "date": "2019-02-11", "pcs": "3"},
    {"number": "6", "dest": "DEA", "date": "2019-03-01", "pcs": "5"}
   ]
  </code></pre>