fix test: also has backup registered

Author: tijmenbruggeman

test/unit/TinySettingsAdminTest.php

@@ -18,6 +18,7 @@ public function test_admin_init_should_register_keys() {
 			array( 'tinify', 'tinypng_compression_timing' ),
 			array( 'tinify', 'tinypng_sizes' ),
 			array( 'tinify', 'tinypng_resize_original' ),
+			array( 'tinify', 'tinypng_backup' ),
 			array( 'tinify', 'tinypng_preserve_data' ),
 			array( 'tinify', 'tinypng_convert_format' ),
 			array( 'tinify', 'tinypng_logging_enabled' ),