Issue #3226944: REST's Request handler doesn't resolve $data argument for put and delete methods.
requested to merge issue/drupal-3226944:3226944-resolve-data-argument-for-put-and-delete-methods into 9.3.x
Changed condition so, that arguments data/unserialized are properly resolved for put and delete methods of REST resource plugins. Refactoring tests in \Drupal\Tests\rest\Kernel\RequestHandlerTest to avoid code duplication and to allow test basic behavior of \Drupal\rest\RequestHandler::handle and $data argument resolution separately.