tests: fix incorrect variable in pipeline.t (openresty#246)

Author: RunnningPig

t/pipeline.t

@@ -160,13 +160,13 @@ flushall: OK
  return
  end
 
- res, err = red:set("dog", "an aniaml")
+ ok, err = red:set("dog", "an aniaml")
  if not ok then
  ngx.say("failed to set dog: ", err)
  return
  end
 
- ngx.say("set result: ", res)
+ ngx.say("set result: ", ok)
 
  local res, err = red:get("dog")
  if not res then