fix: patch up nested groups issue

Author: mychidarko

src/Router.php

@@ -61,18 +61,17 @@ public static function mount(string $path, $handler)
         );
 
         $namespace = static::$namespace;
-        $groupRoute = static::$groupRoute;
 
         if ($groupOptions['namespace']) {
             static::$namespace = $groupOptions['namespace'];
         }
 
-        static::$groupRoute = $path;
+        static::$groupRoute = static::$groupRoute . (strpos($path, '/') !== 0 ? "/$path"  : $path);
 
         call_user_func($handler);
 
         static::$namespace = $namespace;
-        static::$groupRoute = $groupRoute;
+        static::$groupRoute = '';
     }
 
     /**