"logstash-output-gelf" plugin not working with "logstash-5.0.0"

I'm trying to forward a log file by using logstash-5.0.0 with the logstash-output-gelf-3.1.1 plugin on Windows 7 and get the following error:

[2016-10-28T14:52:17,756][INFO ][logstash.pipeline        ] Pipeline main started
[2016-10-28T14:52:17,817][INFO ][logstash.agent           ] Successfully started Logstash API endpoint {:port=>9600}
[2016-10-28T14:52:18,594][FATAL][logstash.runner          ] An unexpected error occurred! {:error=>#<NameError: no method 'debug' for arguments (org.jruby.RubyArray,org.jruby.RubyHash) on Java::OrgApacheLoggingLog4jCore::Logger
  available overloads:
    (org.apache.logging.log4j.Marker,java.lang.String,java.lang.Object[])
    (org.apache.logging.log4j.Marker,java.lang.String,org.apache.logging.log4j.util.Supplier[])
    (java.lang.String,org.apache.logging.log4j.util.Supplier[])
    (java.lang.String,java.lang.Object[])>, :backtrace=>["C:/SDKs/logstash-5.0.0/logstash-core/lib/logstash/logging/logger.rb:41:in `debug'", "C:/SDKs/logstash-5.0.0/vendor/bundle/jruby/1.9/gems/logstash-output-gelf-3.1.1/lib/logstash/outputs/gelf.rb
:190:in `receive'", "C:/SDKs/logstash-5.0.0/logstash-core/lib/logstash/outputs/base.rb:92:in `multi_receive'", "org/jruby/RubyArray.java:1613:in `each'", "C:/SDKs/logstash-5.0.0/logstash-core/lib/logstash/outputs/base.rb:92:in `multi_receive'", "C:/S
DKs/logstash-5.0.0/logstash-core/lib/logstash/output_delegator_strategies/legacy.rb:19:in `multi_receive'", "C:/SDKs/logstash-5.0.0/logstash-core/lib/logstash/output_delegator.rb:42:in `multi_receive'", "C:/SDKs/logstash-5.0.0/logstash-core/lib/logst
ash/pipeline.rb:297:in `output_batch'", "org/jruby/RubyHash.java:1342:in `each'", "C:/SDKs/logstash-5.0.0/logstash-core/lib/logstash/pipeline.rb:296:in `output_batch'", "C:/SDKs/logstash-5.0.0/logstash-core/lib/logstash/pipeline.rb:252:in `worker_loo
p'", "C:/SDKs/logstash-5.0.0/logstash-core/lib/logstash/pipeline.rb:225:in `start_workers'"]}

My .conf file looks like this:

input { 
  file {...}
}

output {
  gelf {
    host => "199.99.99.179"
    port => 12203
  }
}

It seems to be caused by a version mismatch between logstash and the logstash-output-gelf plugin as the same configuration works fine with logstash-2.4.0.
Perhaps the output plugin needs to be updated for 5.0.0?

we changed the logging infrastructure in 5.0.0 to log4j2, and while we did work to mimmic the API, apparently the previous logging framework (Cabin) allowed the logging of an array object, while the new one doesn't (see https://github.com/logstash-plugins/logstash-output-gelf/blob/master/lib/logstash/outputs/gelf.rb#L190)

This should be easy to fix, replacing
@logger.debug(["Sending GELF event", m])
with
@logger.debug("Sending GELF event", :event => m)

I opened https://github.com/logstash-plugins/logstash-output-gelf/issues/20 for this problem