fix mutate specs (broken when grok defaulted singles=>true)
fix mutate specs (broken when grok defaulted singles=>true)

tinglinux/logstash-kafka,joekiller/logstash-kafka,opentable/logstash-kafka,Aloomaio/logstash-kafka,Aloomaio/logstash-kafka,opentable/logstash-kafka,tinglinux/logstash-kafka,joekiller/logstash-kafka