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.
Always use UTF-8 for output with IRC encoding.