type WriterFuncs struct {
    SignInPageFunc func(rw http.ResponseWriter, req *http.Request, redirectURL string, statusCode int)
    ErrorPageFunc  func(rw http.ResponseWriter, opts ErrorPageOpts)
    ProxyErrorFunc func(rw http.ResponseWriter, req *http.Request, proxyErr error)
    RobotsTxtfunc  func(rw http.ResponseWriter, req *http.Request)