title: SDImageCache - imageFromMemoryCache returns nil
body: <p>I'm using the latest version of SDWebImage (SDImageCache), and saving image by code</p>
  
  <pre><code>[[SDImageCache sharedImageCache] storeImage:image forKey:@"1"];
  </code></pre>
  
  <p>and I'm sure the image have been saved successfully, because I've checked my app folder, there is an image existing in it.</p>
  
  <p>and I'm using </p>
  
  <pre><code>UIImage *image = [[SDImageCache sharedImageCache] imageFromMemoryCacheForKey:@"1"];
  </code></pre>
  
  <p>to read my image out, but the image object returns nil. 
  What could the problem be? Thank you!</p>