Write encoding error to error log

Ignore Encoding::UndefinedConversionError, Encoding::InvalidByteSequenceError( If Invalid UTF-8 char in record)

### Outpu Example

````
2013-12-12 17:39:08 +0900 [warn]: "\xE3" from ASCII-8BIT to UTF-8, ignored error_class=Encoding::UndefinedConversionError tag="access" record="{\"status\"=>\"200\", \"request_time\"=>\"0.001\", \"remote_addr\"=>\"127.0.0.1\", \"upstream_addr\"=>\"127.0.0.1:7080\", \"upstream_response_time\"=>\"0.001\", \"request_http_method\"=>\"GET\", \"request_url\"=>\"/get.php?num=52a976294\", \"request_http_version\"=>\"HTTP/1.1\", \"http_referer\"=>\"http://example.com?word=%E3%83%91%E3%83%B3%E3%83%86%E3%82%A3&%E3%82%B9%E3%83%88%E3%83%83%E3%82%AD%E3%83%B3%E3%82%B0with%E3%82%AC%E3%83%BC%E3%82%BF%E3%83%BC%E3%83%99%E3%83%AB%E3%83%88\", \"num\"=>\"52a976294\", \"\\xE3\\x82\\xB9\\xE3\\x83\\x88\\xE3\\x83\\x83\\xE3\\x82\\xAD\\xE3\\x83\\xB3\\xE3\\x82\\xB0with\\xE3\\x82\\xAC\\xE3\\x83\\xBC\\xE3\\x82\\xBF\\xE3\\x83\\xBC\\xE3\\x83\\x99\\xE3\\x83\\xAB\\xE3\\x83\\x88\"=>\"\"}"
````