Table: fix table column show-overflow-tooltip bug

Table: fix table column show-overflow-tooltip bug, low version Firefox without innerText, add textContent

packages/table/src/table-body.js

@@ -343,7 +343,7 @@ export default {
       if (hasClass(cellChild, 'el-tooltip') && cellChild.scrollWidth > cellChild.offsetWidth && this.$refs.tooltip) {
         const tooltip = this.$refs.tooltip;
 
-        this.tooltipContent = cell.innerText;
+        this.tooltipContent = cell.textContent || cell.innerText;
         tooltip.referenceElm = cell;
         tooltip.$refs.popper && (tooltip.$refs.popper.style.display = 'none');
         tooltip.doDestroy();