prevent upercase/downcase for not String object

[adamw-linadm]

I'm hitting this bug:

{:timestamp=>"2014-11-15T19:52:45.395000+0000", :message=>"Exception in filterworker", "exception"=>#>, "backtrace"=>["/opt/logstash/lib/logstash/filters/mutate.rb:338:in lowercase'", "org/jruby/RubyArray.java:1613:ineach'", "/opt/logstash/lib/logstash/filters/mutate.rb:338:in lowercase'", "org/jruby/RubyArray.java:1613:ineach'", "/opt/logstash/lib/logstash/filters/mutate.rb:336:in lowercase'", "/opt/logstash/lib/logstash/filters/mutate.rb:212:infilter'", "(eval):391:in initialize'", "org/jruby/RubyProc.java:271:incall'", "/opt/logstash/lib/logstash/pipeline.rb:262:in filter'", "/opt/logstash/lib/logstash/pipeline.rb:203:infilterworker'", "/opt/logstash/lib/logstash/pipeline.rb:143:in `start_filters'"], :level=>:error}

in logstash 1.4.2. root@logstash-s1:/var/log/logstash# zgrep -c "Exception in filterworker" logstash.log..gz logstash.log.1.gz:213 logstash.log.2.gz:325 logstash.log.3.gz:278 logstash.log.4.gz:32 logstash.log.5.gz:118 logstash.log.6.gz:1 root@logstash-s2:/var/log/logstash# zgrep -c "Exception in filterworker" logstash.log..gz logstash.log.1.gz:220 logstash.log.2.gz:267 logstash.log.3.gz:320 logstash.log.4.gz:53 logstash.log.5.gz:29

When this issue appears logstash freeze and not process new logs. This simple fix resolved this issue.

Ported from logstash 1.4.2 PR https://github.com/elasticsearch/logstash/pull/2091

[jsvd]

Would you mind adding a test for this patch, @adamw-wikia ?

[suyograo]

@adamw-wikia any updates on adding tests? do you need help with the specs?

[purbon]

I will close this PR in benefit of #49 that fix this issue and include tests.

[purbon]

Thanks a lot for your time and contribution.