prevent upercase/downcase for not String object

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