Decode inserts malformed date header

[brosander]

https://github.com/http-rs/async-h1/blob/dffae5fb497dbead73c3d2c87f4d5f9e6e75b2b3/src/client/decode.rs#L74

I'm seeing date header values like

Header date -> [
        "date: Wed, 01 Sep 2021 17:42:04 GMT\r\n",
]

Where date: and \r\n are both included in the value of the header.

I think

   if res.header(DATE).is_none() {
        let date = fmt_http_date(std::time::SystemTime::now());
        res.insert_header(DATE, &format!("date: {}\r\n", date)[..]);
    }

Should be

   if res.header(DATE).is_none() {
        let date = fmt_http_date(std::time::SystemTime::now());
        res.insert_header(DATE, &format!("{}", date)[..]);
    }

[brosander]

Other headers for reference

Header date -> [
        "date: Wed, 01 Sep 2021 17:42:04 GMT\r\n",
    ]
Header content-type -> [
        "application/json",
    ]
Header content-length -> [
        "211",
    ]

[yoshuawuyts]

@brosander ooph, yes that's a great catch! Your suggested fix also makes a lot of sense. Would you be willing to file a PR implementing that?