fix: io, web consistent Completer.complete

[defuncart]

Hi @konsultaner, thanks for creating this package.

I noticed in lib/src/transport/websocket/websocket_transport_io.dart, complete is used from lib/src/transport/abstract_transport.dart

complete(_onDisconnect, error);

complete is also used in lib/src/transport/websocket/websocket_transport_web.dart to close _onConnectionLost, _onDisconnect.

This PR aligns websocket_transport_io to use complete over manual completion, e.g.

-   _onConnectionLost!.complete();
+   complete(_onConnectionLost, null);

In the case of _onConnectionLost!.complete() being called multiple times, this results in Unhandled Exception: Bad state: Future already completed errors as a completer should only be completed once. The existing method complete explicitly checks if the completer has already been completed before invoking complete.