Fix Filetering Select Picklist overwrite

Not able to overwrite dropdown from version 3.2, because of default here https://github.com/railsadminteam/rails_admin/blob/b09f770d9f2b743626d5f29ac9449dbcce9e5c70/src/rails_admin/filtering-select.js#L300

Author: upendra-azad

src/rails_admin/filtering-select.js

@@ -294,10 +294,12 @@ import I18n from "./i18n";
       };
     },
 
-    destroy: function () {
+    destroy: function (isDefault=true) {
       this.input.remove();
       this.button.remove();
-      this.element.html($('<option value="" selected="selected"></option>'));
+      if (isDefault) {
+        this.element.html($('<option value="" selected="selected"></option>'));
+      }
       this.element.show();
       this.filtering_select.remove();
       $.Widget.prototype.destroy.call(this);