Single on/off checkbox is always checked

Problem/Motivation

When you expose a field of type boolean is always checked.

case 'bef_single':
          $show_apply = TRUE;

          // Use filter label as checkbox label.
          $form[$field_id]['#title'] = $filters[$label]->options['expose']['label'];
          $form[$field_id]['#return_value'] = 1;
          $form[$field_id]['#type'] = 'checkbox';
          break;

To reproduce yon can add new field boolean to content type article par exemple, and create new view with this field exposed as checkbox.

See older issue https://www.drupal.org/project/better_exposed_filters/issues/2618500

Proposed resolution

We need to assign correct value after submit.