I think instead of a mutable GeminiRespone, add that not_found() (and subsequently new response types) into GeminiResponse impl and construct one when needed. A

I think instead of a mutable GeminiRespone, add that not_found() (and subsequently new response types) into GeminiResponse impl and construct one when needed. And then, for consistency, GeminiResponse::new() should become GeminiReponse::success().