RuntimeError occurs when Nick is set to ENV['USER']

RuntimeError occurs when Nick is set to ENV['USER'].

    14/01/16-16:22:09 [NDK] Exception RuntimeError - can't modify frozen String
    14/01/16-16:22:09 [NDK] -- backtrace --
    14/01/16-16:22:09 [NDK] | /usr/home/shugo/nadoka/ndk/config.rb:509:in `force_enc
    oding'
    14/01/16-16:22:09 [NDK] | /usr/home/shugo/nadoka/ndk/config.rb:509:in `block in 
    log_format_message'
    14/01/16-16:22:09 [NDK] | /usr/home/shugo/nadoka/ndk/config.rb:486:in `gsub'
    14/01/16-16:22:09 [NDK] | /usr/home/shugo/nadoka/ndk/config.rb:486:in `log_forma
    t_message'
    14/01/16-16:22:09 [NDK] | /usr/home/shugo/nadoka/ndk/config.rb:467:in `log_forma
    t'
    14/01/16-16:22:09 [NDK] | /usr/home/shugo/nadoka/ndk/logger.rb:32:in `log_format
    '
    14/01/16-16:22:09 [NDK] | /usr/home/shugo/nadoka/ndk/logger.rb:69:in `logging'
    14/01/16-16:22:09 [NDK] | /usr/home/shugo/nadoka/ndk/logger.rb:230:in `clog_msgo
    bj'
    14/01/16-16:22:09 [NDK] | /usr/home/shugo/nadoka/ndk/logger.rb:259:in `logging'
    14/01/16-16:22:09 [NDK] | /usr/home/shugo/nadoka/ndk/server.rb:670:in `send_to_c
    lients_otherwise'
    14/01/16-16:22:09 [NDK] | /usr/home/shugo/nadoka/ndk/client.rb:237:in `send_from
    _client'
    14/01/16-16:22:09 [NDK] | /usr/home/shugo/nadoka/ndk/client.rb:57:in `start'
    14/01/16-16:22:09 [NDK] | /usr/home/shugo/nadoka/ndk/server.rb:368:in `block (2 
    levels) in start_clients_thread'
    14/01/16-16:22:09 [NDK] Client Shugo Maeda@<my IP address> disconnected.

log_format_message should copy m before calling force_encoding to avoid `RuntimeError - can't modify f
rozen String' when Nick is set to ENV['USER'], which is frozen.