Fix ResponseHeaderFields.ToString() to emit Connection header from property instead of hardcoding "Upgrade"

[Copilot]

ResponseHeaderFields.ToString() hardcoded Connection: Upgrade whenever Upgrade was set, making the Connection property effectively unused in HTTP output. This broke any response that set Connection to a value other than "Upgrade", and silently dropped the Connection header for any response that didn't also set Upgrade.

Changes

• ResponseHeaderFields.cs: Decouple Connection and Upgrade header emission — each is now written independently when its property is non-empty. Connection: Upgrade is no longer hardcoded.

ResponseSwitchingProtocols already explicitly sets Header.Connection = connection, so WebSocket 101 handshakes continue to emit the correct headers without any changes to the caller.

// Before: Connection: Upgrade always hardcoded when Upgrade is set
if (!string.IsNullOrWhiteSpace(Upgrade))
{
    sb.AppendLine("Upgrade: " + Upgrade);
    sb.AppendLine("Connection: Upgrade"); // ignores Connection property
}

// After: each header driven by its own property
if (!string.IsNullOrWhiteSpace(Connection))
    sb.AppendLine("Connection: " + Connection);

if (!string.IsNullOrWhiteSpace(Upgrade))
    sb.AppendLine("Upgrade: " + Upgrade);