Case issue when upgrading to websockets

[nicolaspernoud]

Hello,

Thanks for this amazing crate.
I have an error when proxying ttyd : "backend tried to switch to protocol Some("WebSocket") when Some("websocket") was requested"... It would seems that converting all to lowercase like this :

fn get_upgrade_type(headers: &HeaderMap) -> Option<String> {
    if headers
        .get(&*CONNECTION_HEADER)
        .map(|value| {
            value
                .to_str()
                .unwrap()
                .split(',')
                .any(|e| e.trim().to_lowercase() == "upgrade")
        })
        .unwrap_or(false)
    {
        if let Some(upgrade_value) = headers.get(&*UPGRADE_HEADER) {
            debug!(
                "Found upgrade header with value: {}",
                upgrade_value.to_str().unwrap().to_owned()
            );

            return Some(upgrade_value.to_str().unwrap().to_lowercase().to_owned());
        }
    }

    None
}

would do the trick.

If it is OK for you I can make a PR, or let you change it directly, as convenient for you.

Best regards.