fix toolbar dropdown button border

src/themes/bulma/bootstrap-table-bulma.scss

@@ -25,17 +25,19 @@
       margin-right: 0;
     }
 
-    .keep-open.button.dropdown {
+    .button.dropdown {
       padding: 0;
       margin: 0;
       border: 0;
 
-      .button {
+      &:not(:first-child) .button {
         border-bottom-left-radius: 0;
         border-top-left-radius: 0;
+      }
+
+      &:last-child .button {
         border-bottom-right-radius: 4px;
         border-top-right-radius: 4px;
-        margin-right: 0;
       }
 
       .dropdown-content {