Merge pull request #421 from cosmok/mutate_add_debug

debug msg when trying to split a non-string type

lib/logstash/filters/mutate.rb

@@ -323,6 +323,9 @@ class LogStash::Filters::Mutate < LogStash::Filters::Base
     @split.each do |field, separator|
       if event[field].is_a?(String)
         event[field] = event[field].split(separator)
+      else 
+        @logger.debug("Can't split something that isn't a string",
+                      :field => field, :value => event[field])
       end
     end
   end