title: NSURLCache returning nil when trying to access cached request from disk
body: <p>I am trying to use the disk based url cacing built in to iOS 5. I have a json feed which i want to load and then cache, so its instantly loaded the next time a user uses the app. 
  The cache.db file is successfully created and if i open it in a sqlite-editor i can fetch out the data.</p>
  
  <p>I am using AFNetworking</p>
  
  <pre><code>NSURL *url = [NSURL URLWithString:@"http://myurl.com/json"];
  NSMutableURLRequest *request = [NSMutableURLRequest requestWithURL:url];
  
  NSLog(@"%d before call",[[NSURLCache sharedURLCache] currentDiskUsage]);  // logs 0 bytes
  AFJSONRequestOperation *operation = [AFJSONRequestOperation JSONRequestOperationWithRequest:request success:^(NSURLRequest *request, NSHTTPURLResponse *response, id JSON) {
  // is successfully loading JSON and reading it to a table view in a view
  }failure:nil];
  
  NSCachedURLResponse *cachedResponse = [[NSURLCache sharedURLCache] cachedResponseForRequest:request] 
  // this returns nil, and can't load the request from disk.
  </code></pre>
  
  <p>In the cache response block, strangely the cache is now working, and successfully gets the cacheurl request from memory.</p>
  
  <pre><code>[operation setCacheResponseBlock:^NSCachedURLResponse *(NSURLConnection *connection, NSCachedURLResponse *cachedResponse) {
          NSCachedURLResponse *cachedResponse_ = [[NSURLCache sharedURLCache] cachedResponseForRequest:request];
          NSLog(@"%d AFTER CACHE",[[NSURLCache sharedURLCache] currentDiskUsage]);
          // this writes out 61440
          return cachedResponse;
      }];
  </code></pre>
  
  <p>How can i load the cached URL request from disk the next time i start the app?</p>