Add args from 2nd to fluent.** internal messages

Currently, when fluentd calls logger like ([out_forwrd.rb#L397](https://github.com/fluent/fluentd/blob/master/lib/fluent/plugin/out_forward.rb#L397))

```
@log.warn "detached forwarding server '#{@name}'", :host=>@host, :port=>@port, :hard_timeout=>true
```

although the logger outputs to the log file like,

```
2014-03-12 12:38:59 +0900 [warn]: detached forwarding server 'host1:22000' host="host1" port=22000 hard_timeout=true
````

fluent.warn message becomes as followings:

```json
{"message":"detached forwarding server 'host1:22000'"}
```

It is losing information, and it made me confuse. I prefer to have the same information with the log. 

This patch fixes the issue. 

I checked all log method calling, and it looked the volume to be added is not huge.