title: NSURLSessionDownloadTask cancelByProducingResumeData return null
body: <p>I use NSURLSessionDownloadTask to download a file from server, it is run in a background session. I want to save resume data to resume the download when the app is terminated. However, the call of cancelByProducingResumeData always returns null. I cannot figure out the issue, cause I used to do a sample of resumable download task successful before. Does anybody get the same issue?</p>