Use ISO-8859-1 instead of CP1252 for sending messages.

Apparently, some IRC clients don't support CP1252 messages, from which I
confirmed HexChat and Quassel IRC, but it's likely there are more IRC
clients that don't support CP1252 (by the way, CP1252 is not
ISO-8859-1).

Following the robustness principle, the IRC encoding was updated to
send messages using ISO-8859-1, but read them using CP1252. ISO-8859-1,
unlike CP1252 is supported practically everywhere.

This will cause a slight incompatibility issue with the clients that
don't support UTF-8, but support CP1252 (not just ISO-8859-1). In my
opinion it's better to make those rare messages readable for modern
IRC clients, rather than old versions of IRC clients such as mIRC 6.16
released 10 years ago.