Fix @source_host when it comes from json_event
Fix @source_host when it comes from json_event

This is due to the change 1fc568e07d957ab9b629eab7550a1ae190db3c8a
which tries to get the @source value set sanely for json_event events which don't
have one.

However Logstash::Event unilaterally resets the @source_host when @source is set, which
is not what we want, at all. I have changed this so that @source_host is only set from
source in the case where @source_host was previously unset

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