Amazon_es output plugin routing

```I am using amazon_ws output to send data, it works perfectly without `routing` defined but throws an error `illegal_argument_exception\",\"reason\":\"Action/metadata line [1] contains an unknown parameter [_routing]\"},\"status\":400}"}` when i try to rout documents to  specifig shards my output code looks like 

output {
	amazon_es {
	  hosts => "${LOGSTASH_ELASTICSEARCH_HOST}"
    index => "inspectionrds_workorder"
    region => "us-east-1"
	  ssl => true
    document_id => "workorder_%{work_order_id}"
    routing => "%{work_order_id}"
	}
	stdout { codec => rubydebug }
}```