title: SDWebImage not caching
body: <p>I'm downloading images using [SDWebImageDownloader.sharedDownloader downloadImageWithURL] with options set to 0.  I'm initially not doing anything with them, with the understanding that they will be cached.  However, when I use the exact same function to later display an image, the function is downloading the image again, rather than getting it from the cache (the image cache type is 0). In both cases, the url of the image is the same.  Is my understanding regarding caching incorrect?</p>