修复多行表头在 IE 中缺少左边框问题 (#3901)

src/bootstrap-table.js

@@ -825,7 +825,8 @@
             Utils.sprintf(' rowspan="%s"', column.rowspan),
             Utils.sprintf(' colspan="%s"', column.colspan),
             Utils.sprintf(' data-field="%s"', column.field),
-            j === 0 && column.fieldIndex ? ' data-not-first-th' : '',
+            // If `column` is not the first element of `this.options.columns[0]`, then className 'data-not-first-th' should be added.
+            j === 0 && i > 0 ? ' data-not-first-th' : '',
             '>')
 
           html.push(Utils.sprintf('<div class="th-inner %s">', this.options.sortable && column.sortable