title: Redis mocha Test case issue
body: <p>I have one file call cache.js</p>
  
  <pre><code>var redisCache = redis.createClient(port, name);
  redisCache.on("error", function(err) {
    logger.error("Error connecting to redis", err);
  });
  
  exports.setExp = function(key, timeLeft, data){
      redisCache.set(key, JSON.stringify(data), function (err, reply) {
        console.log("error "+err);
        console.log("reply "+reply);
        if(err) {
          console.log("error "+err.command + err.code);
          logger.info("This errror on set key related to node_redis");
        }
        if(reply == 'OK') {
          redisCache.expire(key, timeLeft, function (err, reply) {
            if(err) {
              logger.info("This errror on expire key related to node_redis");
            }
            if(reply === 1) {
              logger.info(key+" key expire time set as "+timeLeft+" successfully!");
            }
          });
        }
      });
    }
  </code></pre>
  
  <p>Now I want to write the test case for the above setExp function but some how the node_redis aways return me the err as <code>null</code> and reply as <code>OK</code></p>
  
  <p>below is my test case. </p>
  
  <pre><code>var cache = require(path.join(__dirname,'..','/cache'));
  describe('cache', function () {
    it('Cache #setExp() ', function (done) {
        var result = cache.setExp(undefined, 0, []);
          assert.equal('OK', results);
          done()
    })
  })
  </code></pre>
  
  <p>IF I change the it should follow the below error I mention as per the <a href="https://github.com/NodeRedis/node_redis/blob/master/test/commands/set.spec.js" rel="nofollow noreferrer">node_redis test case</a></p>
  
  <pre><code>var result = cache.setExp('foo', 10, []);
  </code></pre>
  
  <p>it should return me the error called ERR wrong number of arguments for 'set' command</p>
  
  <pre><code>var result = cache.setExp(undefined, 0, []);
  </code></pre>
  
  <p>It should accept the below error log as 
      assert.equal(err.command, 'SET');</p>
  
  <p>Please suggest me right way to achieve this. </p>