Add Response::from_res

[yoshuawuyts]

This adds Response::from_res which enables us to bridge with Surf for authoring proxies conveniently. We couldn't add this as a generic From bound because it would conflict with existing implementations.

This was in response to this this example which proxies from surf -> tide, which would allow us to rewrite the core logic from ~10 lines to 2 lines.

let res = surf::get(...).await?;
Ok(Response::from_res(res))

[chrisdickinson]

Would it be possible to have a form of this where we could pass a HashMap to override headers on the way through?

Response::from_res(res, hashmap!{
    "ETag" => "a modified etag",
})

(Or possibly a lambda to modify them?)

Response::from_res(res, |headers: &mut Headers| {
  // edit the headers on the way through
})

[yoshuawuyts]

The following would be possible with this patch:

let res = surf::get(...).await?;
let mut res = Response::from_res(res);
res.set_header("etag".parse()?, "<new etag>");
Ok(res)

In future patches well likely rename set_header to insert_header, and remove the need for calling parse()?. But the overall approach should remain the same.