[BrowserKit] fix return type declarations

src/Symfony/Component/BrowserKit/Client.php

@@ -141,9 +141,9 @@ abstract class Client
      * Gets single server parameter for specified key.
      *
      * @param string $key     A key of the parameter to get
-     * @param string $default A default value when key is undefined
+     * @param mixed  $default A default value when key is undefined
      *
-     * @return string A value of the parameter
+     * @return mixed A value of the parameter
      */
     public function getServerParameter($key, $default = '')
     {