Detect parent operation cancellation in download operation completion block
Merge pull request #311 from sebreh/master

Detect cancelled parent operation in download operation callback
Merge remote-tracking branch 'upstream/master'

* upstream/master:
  Detect parent operation cancellation in download operation completion block
  Prefetching file properties in the disk cleaning enumerator
  correct cachePolicy constants.
  Add (kinda) LIFO queue mode support for image downloading (fix #294)
  Add completion block support to `SDWebImagePrefetcher` (fix #127)
  Add `SDWebImageManager` delegate allowing fine control of manager's cache-in
  Add ability to set custom downloader HTTP headers (fix #171)
  Synchronize access to SDWebImageManager's mutable structures (fix #301)
  [BUG] Fix invalid alpha on JPEG files
Merge branch 'master' of https://github.com/Kicksend/SDWebImage

* 'master' of https://github.com/Kicksend/SDWebImage: