Fix CLANG_WARN_OBJC_IMPLICIT_RETAIN_SELF warning
Fix CLANG_WARN_OBJC_IMPLICIT_RETAIN_SELF warning

iheartradio/SDWebImage,dreampiggy/SDWebImage,zhang-yawei/SDWebImage,dreampiggy/SDWebImage,awkward/SDWebImage,awkward/SDWebImage,dreampiggy/SDWebImage,smule/SDWebImage,Insfgg99x/SDWebImage,rs/SDWebImage,awkward/SDWebImage,tualatrix/SDWebImage