Don't include identity in Content-Encoding header

[Bobo1239]

According to the spec identity should only be used for the Accept-Encoding header and not be included in Content-Encoding.

In practice this change fixes chunked video loading in Firefox when using ServeDir. (making seeking possible without downloading the whole video file)

[Nehliin]

Thanks for the PR! Do you have an example when this actually is set as an header value? I updated some of our tests to check if we return the encoding when serving uncompressed files and it doesn't look like we set the header value in the basic scenarios.

[Bobo1239]

Thanks for the review. I've added a test which would've returned Content-Encoding: identity previously.

Also for posterity's sake: The original problem (video seeking) can be reproduced by using the simplest ServeDir example (see below) and navigating to a large video file in Firefox. In my case this was a 1.2GB .webm video. Without this PR seeking inside the video had a noticeable delay as the whole video would be downloaded and no range requests would be used. (not sure whether Firefox should do better here or not)

Code

use std::net::{Ipv4Addr, SocketAddr};

use axum::Server;
use tower::make::Shared;
use tower_http::services::ServeDir;

#[tokio::main]
async fn main() {
    let addr = SocketAddr::from((Ipv4Addr::UNSPECIFIED, 3000));

    let service = ServeDir::new(".");
    Server::bind(&addr)
        .serve(Shared::new(service))
        .await
        .expect("server error");
}

[Nehliin]

Thanks!