allow logging of frozen strings

after integrating this gem with our sinatra apps, we had test failures related to frozen strings. specifically the "env['sinatra.error'].message" string in an "error" block

     RuntimeError:
       can't modify frozen string
     # /Users/kthornhill/.rvm/gems/ruby-1.9.2-p318/gems/yell-adapters-syslog-1.2.0/lib/yell/adapters/syslog.rb:173:in `strip!'

this PR checks the frozen state of the message and duplicates it if needed.