fix(DropdownItem): missing default active color (#4330)

src/dropdown-item/index.js

@@ -82,7 +82,7 @@ export default createComponent({
           key={option.value}
           icon={option.icon}
           title={option.text}
-          class={bem('option')}
+          class={bem('option', { active })}
           style={{ color: active ? activeColor : '' }}
           onClick={() => {
             this.showPopup = false;

src/dropdown-item/index.less

@@ -8,6 +8,14 @@
 
   &__option {
     text-align: left;
+
+    &--active {
+      color: @dropdown-menu-option-active-color;
+
+      .van-dropdown-item__icon {
+        color: @dropdown-menu-option-active-color;
+      }
+    }
   }
 
   &--up {