'internal error: missing mime' when echoing body without Content-Type

[tirr-c]

async_h1::accept panics with the following endpoint and request:

|req| async move {
    let mut res = Response::new(StatusCode::Ok);
    res.set_body(req);
    Ok(res)
}

POST / HTTP/1.1
content-length: 0

[tirr-c]

Related PR: http-rs/http-types#83