Fix background download operation
Fix background download operation

FIx background download

Add task check when operation will deallocated && tidy code

dreampiggy/SDWebImage,rs/SDWebImage,awkward/SDWebImage,iheartradio/SDWebImage,tualatrix/SDWebImage,awkward/SDWebImage,dreampiggy/SDWebImage,dreampiggy/SDWebImage,zhang-yawei/SDWebImage,awkward/SDWebImage