in_sample: duplicate record to support destructive change

When in_sample plugin is used with filter parser which uses
remove_key_name_field, it raises the following error repeatedly.

  #0 dump an error event: error_class=ArgumentError error="message
  does not exist"

This kind of error occurs when key_name and remove_key_name_field
removes key from record with destructive change in filter parser
affects generated sample data.

ref. #4575

Signed-off-by: Kentaro Hayashi <hayashi@clear-code.com>

lib/fluent/plugin/in_sample.rb

@@ -118,14 +118,13 @@ def emit(num)
     end
 
     def generate
-      d = @sample[@sample_index]
+      d = @sample[@sample_index].dup
       unless d
         @sample_index = 0
-        d = @sample[@sample_index]
+        d = @sample[@sample_index].dup
       end
       @sample_index += 1
       if @auto_increment_key
-        d = d.dup
         d[@auto_increment_key] = @storage.update(:auto_increment_value){|v| v + 1 }
       end
       d