Set Redis keys only if they don't exist (#156)

Author: AlexDWu

lib/classifier-reborn/backends/bayes_redis_backend.rb

@@ -34,8 +34,8 @@ def initialize(options = {})
       end
 
       @redis = Redis.new(options)
-      @redis.set(:total_words, 0)
-      @redis.set(:total_trainings, 0)
+      @redis.setnx(:total_words, 0)
+      @redis.setnx(:total_trainings, 0)
     end
 
     def total_words