title: Enyim Memcached client doesn't work with expiration parameter passed
body: <p>when using Enyim memcached client to store data with a expire timespan, i find that it doesn't work. 
  Could anyone help?</p>
  
  <p>In my test code, I store the date with 10 minutes expiration time, and i try to get it from cache immediately but got a null object.</p>
  
  <pre><code>Enyim.Caching.MemcachedClient client = new Enyim.Caching.MemcachedClient();
  client.Store(Enyim.Caching.Memcached.StoreMode.Set, "one", 1, new TimeSpan(0, 10, 0));
  
  object obj;
  client.TryGet("one", out obj); // obj == null
  
  Assert.AreNotEqual(null, obj); // this will fail
  </code></pre>