same change of 37fa6698b1227652b0409a0bde0dab7dda5e93f7 to out_kafka.rb
same change of 37fa6698b1227652b0409a0bde0dab7dda5e93f7 to out_kafka.rb

- ref: https://github.com/fluent/fluent-plugin-kafka/pull/196#issuecomment-393415500

Signed-off-by: sawanoboly <66d3c633bfe3cbb91c59a831a86365231a583806@higanworks.com>

htgc/fluent-plugin-kafka