Fixed typo.
SDWebImageDownloaderQueueMode type renamed. Fixed typo. Added description for renamed type. Type renamed because "queue" notion is a FIFO only, but LIFO is a stack, and if we give the type a neutral name, we can avoid logical inconsistencies.
Potential strong cycle fixed.
Merge branch 'develop'

* develop:
  Potential strong cycle fixed.
  SDWebImageDownloaderQueueMode type renamed. Fixed typo. Added description for renamed type. Type renamed because "queue" notion is a FIFO only, but LIFO is a stack, and if we give the type a neutral name, we can avoid logical inconsistencies.
  Fixed typo.