Exception if record contains Time class

If the record contains a time class, the following exception occurs:
```
2017-10-24 20:56:28 +0000 [warn]: emit transaction failed: error_class=NoMethodError error="undefined method `to_msgpack' for 2017-10-24 20:56:12 +0000:Time" tag="mdsd.rails"
2017-10-24 20:56:28 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/fluent-plugin-mdsd-0.1.6.pre.build.master.54/lib/fluent/plugin/out_mdsd.rb:70:in `to_msgpack'
2017-10-24 20:56:28 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/fluent-plugin-mdsd-0.1.6.pre.build.master.54/lib/fluent/plugin/out_mdsd.rb:70:in `format'
2017-10-24 20:56:28 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/fluentd-0.12.40/lib/fluent/output.rb:284:in `block in format_stream'
2017-10-24 20:56:28 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/fluentd-0.12.40/lib/fluent/event.rb:130:in `call'
2017-10-24 20:56:28 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/fluentd-0.12.40/lib/fluent/event.rb:130:in `block in each'
2017-10-24 20:56:28 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/fluentd-0.12.40/lib/fluent/event.rb:129:in `each'
2017-10-24 20:56:28 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/fluentd-0.12.40/lib/fluent/event.rb:129:in `each'
2017-10-24 20:56:28 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/fluentd-0.12.40/lib/fluent/output.rb:283:in `format_stream'
2017-10-24 20:56:28 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/fluentd-0.12.40/lib/fluent/output.rb:269:in `emit'
2017-10-24 20:56:28 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/fluentd-0.12.40/lib/fluent/output.rb:42:in `next'
2017-10-24 20:56:28 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/fluentd-0.12.40/lib/fluent/plugin/out_copy.rb:78:in `emit'
2017-10-24 20:56:28 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/fluentd-0.12.40/lib/fluent/event_router.rb:154:in `emit'
2017-10-24 20:56:28 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/fluentd-0.12.40/lib/fluent/event_router.rb:90:in `emit_stream'
2017-10-24 20:56:28 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/fluentd-0.12.40/lib/fluent/plugin/out_relabel.rb:24:in `emit'
2017-10-24 20:56:28 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/fluentd-0.12.40/lib/fluent/event_router.rb:90:in `emit_stream'
2017-10-24 20:56:28 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/fluentd-0.12.40/lib/fluent/event_router.rb:81:in `emit'
2017-10-24 20:56:28 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/fluent-plugin-rewrite-tag-filter-1.5.6/lib/fluent/plugin/out_rewrite_tag_filter.rb:75:in `block in emit'
2017-10-24 20:56:28 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/fluentd-0.12.40/lib/fluent/event.rb:130:in `call'
2017-10-24 20:56:28 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/fluentd-0.12.40/lib/fluent/event.rb:130:in `block in each'
2017-10-24 20:56:28 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/fluentd-0.12.40/lib/fluent/event.rb:129:in `each'
2017-10-24 20:56:28 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/fluentd-0.12.40/lib/fluent/event.rb:129:in `each'
2017-10-24 20:56:28 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/fluent-plugin-rewrite-tag-filter-1.5.6/lib/fluent/plugin/out_rewrite_tag_filter.rb:72:in `emit'
2017-10-24 20:56:28 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/fluentd-0.12.40/lib/fluent/event_router.rb:154:in `emit'
2017-10-24 20:56:28 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/fluentd-0.12.40/lib/fluent/event_router.rb:90:in `emit_stream'
2017-10-24 20:56:28 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/fluentd-0.12.40/lib/fluent/plugin/in_forward.rb:178:in `on_message'
2017-10-24 20:56:28 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/fluentd-0.12.40/lib/fluent/plugin/in_forward.rb:338:in `call'
2017-10-24 20:56:28 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/fluentd-0.12.40/lib/fluent/plugin/in_forward.rb:338:in `block in on_read_msgpack'
2017-10-24 20:56:28 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/fluentd-0.12.40/lib/fluent/plugin/in_forward.rb:337:in `feed_each'
2017-10-24 20:56:28 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/fluentd-0.12.40/lib/fluent/plugin/in_forward.rb:337:in `on_read_msgpack'
2017-10-24 20:56:28 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/cool.io-1.5.1/lib/cool.io/io.rb:123:in `on_readable'
2017-10-24 20:56:28 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/cool.io-1.5.1/lib/cool.io/io.rb:186:in `on_readable'
2017-10-24 20:56:28 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/cool.io-1.5.1/lib/cool.io/loop.rb:88:in `run_once'
2017-10-24 20:56:28 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/cool.io-1.5.1/lib/cool.io/loop.rb:88:in `run'
2017-10-24 20:56:28 +0000 [warn]: /opt/td-agent/embedded/lib/ruby/gems/2.1.0/gems/fluentd-0.12.40/lib/fluent/plugin/in_forward.rb:120:in `run'
```

This is because the Time class does not support the method "to_msgpack"