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](https://en.wikipedia.org/wiki/Windows-1252) is not [ISO-8859-1](https://en.wikipedia.org/wiki/ISO/IEC_8859-1)).

Following the [robustness principle](https://en.wikipedia.org/wiki/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.