Unrecognized Timestamp should log more detail

If the timestamp is not recognized, then it logs a warning:

```
{:timestamp=>"2016-02-26T07:58:50.187000-0800", :message=>"Unrecognized @timestamp value, setting current time to @timestamp, original in _@timestamp field", :value=>"\"2016-02-26 07:58:38,871\"", :level=>:warn}
```

It would be useful if it logged more detail, such as the type to make it easier to trace for more advanced Logstash use cases:

```
{:timestamp=>"2016-02-26T07:58:50.187000-0800", :message=>"Unrecognized @timestamp value, setting current time to @timestamp, original in _@timestamp field", :value=>"\"2016-02-26 07:58:38,871\"", :type=>"apache", :level=>:warn}
```

It may be easiest to add a meta field for the input source to trace back the actual event failure when multiple inputs are used.