Fix: Converted raw string include NULL character in charset module

(before)
[2013-10-11 12:01:19.863275] charset: Trying to convert [PRIVMSG+%23test+%3A%E3%81%82%E3%81%82%E3%81%82%E3%81%82] from [UTF-8] to [ISO-2022-JP]...
[2013-10-11 12:01:19.864302] charset: Converted: [PRIVMSG+%23test+%3A%1B%24B%24%22%24%22%24%22%24%22%00%00%00] from [UTF-8] to [ISO-2022-JP]!

(after)
[2013-10-11 12:07:09.236879] charset: Trying to convert [PRIVMSG+%23test+%3A%E3%81%82%E3%81%82%E3%81%82%E3%81%82] from [UTF-8] to [ISO-2022-JP]...
[2013-10-11 12:07:09.236984] charset: Converted: [PRIVMSG+%23test+%3A%1B%24B%24%22%24%22%24%22%24%22] from [UTF-8] to [ISO-2022-JP]!