Preserve subsearch on settings change, a=chris

diff --git a/src/controllers/SearchController.php b/src/controllers/SearchController.php
index 91660ebff..357282e38 100755
--- a/src/controllers/SearchController.php
+++ b/src/controllers/SearchController.php
@@ -572,7 +572,7 @@ class SearchController extends Controller implements CrawlConstants
             }
             return $this->redirectWithMessage(
                 tl('settings_controller_settings_saved'),
-                ['return', 'oldc', 'q']);
+                ['return', 'oldc', 'q', 's']);
         } else if (isset($_REQUEST['perpage'])) {
             return $this->redirectWithMessage("",
                 ['return', 'oldc', 'q']);