Implement Clone for AppendHeaders

[jbirnick]

• I have looked for existing issues (including closed) about this

Feature Request

Please implement Clone for AppendHeaders.

Motivation

I need it in my project. I once and forever create an AppendHeaders object which contains some headers that I add to every response. However, when I create a response, the AppendHeaders object is consumed by into_response_parts(...) of IntoResponseParts. So I need to be able to clone it for every response.

Proposal

Add a #[derive(Clone)] to the AppendHeaders struct.

...
#[derive(Clone)]
pub struct AppendHeaders<I>(pub I);

Alternatives

I guess could only store the contents of my AppendHeaders object, which I can clone, and then construct a new AppendHeaders object from cloned contents for every reponse. However, I really want to store the wrapping AppendHeaders object, because that it the correct semantics.

I think it's very natural that AppendHeaders should be clonable (if the contents are clonable), and I'm surprised that it's not already implemented.

[jplatte]

Seems reasonable, and we already derive this for many extractors, as well as the Html response wrappers.
Do you want to send a PR? Deriving Copy would be good as well for consistency.