fix[dragTable]: fixed bug in Firefox

src/views/example/table/dragTable.vue

@@ -110,6 +110,11 @@ export default {
       const el = document.querySelectorAll('.el-table__body-wrapper > table > tbody')[0]
       this.sortable = Sortable.create(el, {
         ghostClass: 'sortable-ghost', // Class name for the drop placeholder,
+        setData: function(dataTransfer) {
+          dataTransfer.setData('Text', '')
+          // to avoid Firefox bug
+          // Detail see : https://github.com/RubaXa/Sortable/issues/1012
+        },
         onEnd: evt => {
           const targetRow = this.list.splice(evt.oldIndex, 1)[0]
           this.list.splice(evt.newIndex, 0, targetRow)