generating document_type=log by default when fields hash present

[sasubillis]

When I use the following prospector section in the manifest file
filebeat::prospector { 'syslog': paths => [ '/var/log/messages','/var/log/secure' ], fields_under_root => true, fields => {'type' => 'syslog'}, }
the generated filebeat prospector config has got document_type: log which is not required.
and filebeat log still complaining about document_type
2018-05-24T00:20:11Z WARN DEPRECATED: document_type is deprecated. Use fields instead.