short_message not appearing

Nobody has claimed this yet.

I'm in the process of testing out log4js-node as a replacement for bunyan, and checking that the gelf transport works as expected.

I'm using gelf to send data to logstash. It's working for the most part, except that `short_message` doesn't appear in kibana. If I change the code to use `_short_message` instead, it works as expected. I've tried several different layouts - currently testing with `dummy`, but I've tested `basic` and the default.

My apender looks like this:

`gelf: {type: '@log4js-node/gelf', host: 'xxx', port: 12201, facility: 'log-test', layout: {type: 'dummy'}, customFields: {_foo: 'bar'}}`

This is really weird. The facility property gets sent correctly, timestamp, host, level etc all work.

According to the [logstash docs](https://www.elastic.co/guide/en/logstash/current/plugins-outputs-gelf.html#plugins-outputs-gelf-custom_fields) `short_message` is expected, and it works with the existing bunyan setup.

I can't see any important difference between the [code in this repo](https://github.com/log4js-node/gelf/blob/master/lib/index.js#L92) vs the [gelf-stream package used for bunyan](https://github.com/mhart/gelf-stream/blob/master/gelf-stream.js#L83).

I also checked gelf-stream's dep [gelfling](https://github.com/mhart/gelfling/blob/master/gelfling.js#L103) and it's not adding the `_`.

I am still looking for configuration issues that could be causing this, but I'm pretty confused because the messages are being sent to a different index but the same server, with all the same config as the working bunyan version.

Any ideas?

Start with lib/index.js around line 92 and reproduce the issue using the shown GELF appender configuration and dummy layout. Compare the generated fields with gelf-stream.js around line 83 and gelfling.js around line 103, focusing on why short_message becomes unavailable while _short_message works. Done means the GELF output exposes short_message correctly in Logstash/Kibana.

Written by the indexing model from the issue text.