The problem in parser.rb

### The Problems I have found for LTSVParser.
* ValuesParser is not delete the output of time key like other do so.
note: ValuesParser was used by TSVParser, TSVParser, CSVParser
* LTSVParser always use current time when `time_key` wasn't set.
This behavior is not same as ApacheParser, JSONParser and RegexpParser.
I think it should be 'time' for the default of `time_key`.

Despite setting time_format, LTSVParser would not use time key. it use current time instead of log output time.
```
# quoted from http://ltsv.org/
<source>
  type tail
  format ltsv
  time_format %d/%b/%Y:%H:%M:%S %z
  path /var/log/nginx/access_log
  pos_file /var/log/nginx/access_log.pos
  tag nginx.access
</source>
```

### My Patch is...
* ValuesParser
Time key would be deleted when it has used for fluentd internal time like TSVParser, TSVParser, CSVParser do so.

* LTSVParser
LTSVParser using `time` for the default of `time_key` like ApacheParser, JSONParser and RegexpParser do so.

After this patch has used, will work fine.