fix: Omit default port from Host header in MqttServerWs2Connection

[fujimonfn]

Summary

The _performWSHandshake method in MqttServerWs2Connection always includes the port number in the Host header, even when using the default port for the scheme (443 for wss://, 80 for ws://).

Per RFC 7230 Section 5.4:

If the target URI includes an authority component, then a client MUST send a field-value for Host that is identical to that authority component, excluding any userinfo subcomponent and its "@" delimiter.

And RFC 3986 Section 3.2.3:

URI producers and normalizers should omit the port component and its ":" delimiter if port is empty or if its value would be the same as that of the scheme's default.

Current behavior: Host: endpoint:443 (always includes port)
Expected behavior: Host: endpoint (omit default port)

Problem

AWS IoT Core (and potentially other WebSocket servers that perform SigV4 or similar Host-header-based authentication) rejects WebSocket upgrade requests with Host: endpoint:443, returning 403 Forbidden. This is because the server normalizes the Host header by stripping the default port, causing a mismatch with the signed request.

The standard MqttServerWsConnection does not have this issue because Dart's WebSocket.connect() / HttpClient correctly omits the default port from the Host header.

Fix

Only include the port in the Host header when it differs from the scheme's default port:

• wss:// → default port 443
• ws:// → default port 80

[shamblett]

Merged, package re published at version 10.11.10.