[BrowserKit][HttpClient][Routing] support building query strings with stringables

Author: nicolas-grekas

Generator/UrlGenerator.php

@@ -295,6 +295,17 @@ protected function doGenerate(array $variables, array $defaults, array $requirem
             return $a == $b ? 0 : 1;
         });
 
+        array_walk_recursive($extra, $caster = static function (&$v) use (&$caster) {
+            if (\is_object($v)) {
+                if ($vars = get_object_vars($v)) {
+                    array_walk_recursive($vars, $caster);
+                    $v = $vars;
+                } elseif (method_exists($v, '__toString')) {
+                    $v = (string) $v;
+                }
+            }
+        });
+
         // extract fragment
         $fragment = $defaults['_fragment'] ?? '';