mod_http: Fix reliance on previous tostring() format of util.set a863e4237b91 unintentionally changed the format of HTTP CORS headers, which were apparently relying on the output of tostring(), which it shouldn't have. Explicitly serializing it this time.

module:log("error", "mod_pep_plus has been renamed to mod_pep, please update your config file. Auto-loading mod_pep...");
module:depends("pep");