Fix LOGSTASH-2164

As discussed in https://logstash.jira.com/browse/LOGSTASH-2164
gsub!() is dangerous to use like this as it returns nil when no substitutions are made.
Replacing with gsub() avoid this.
Comes from https://github.com/elasticsearch/logstash-contrib/pull/52
May I suggest `table_id.tr(':-','_')` instead?