Fluentd's logger does #to_s for each arguments, so we shouldn't do to_s when logger called.
```ruby
# from lib/fluent/log.rb 
def event(level, args)
    time = Time.now
    message = ''
    map = {}
    args.each {|a|
      if a.is_a?(Hash)
        a.each_pair {|k,v|
          map[k.to_s] = v
        }
      else
        message << a.to_s
      end
    }
```