Fix nullability of ASImageDownloaderProtocol to conform to ASBasicImageDownloader behavior
Fix nullability of ASImageDownloaderProtocol to conform to ASBasicImageDownloader behavior

rmls/AsyncDisplayKit,hannahmbanana/AsyncDisplayKit,rcancro/AsyncDisplayKit,romyilano/AsyncDisplayKit,jellenbogen/AsyncDisplayKit,george-gw/AsyncDisplayKit,bimawa/AsyncDisplayKit,nrcmedia/AsyncDisplayKit,eanagel/AsyncDisplayKit,yufenglv/AsyncDisplayKit,lappp9/AsyncDisplayKit,rmls/AsyncDisplayKit,gazreese/AsyncDisplayKit,harryworld/AsyncDisplayKit,tomizimobile/AsyncDisplayKit,garrettmoon/AsyncDisplayKit,aaronschubert0/AsyncDisplayKit,rahul-malik/AsyncDisplayKit,maicki/AsyncDisplayKit,samhsiung/AsyncDisplayKit,chrisdanford/AsyncDisplayKit,hannahmbanana/AsyncDisplayKit,samhsiung/AsyncDisplayKit,JetZou/AsyncDisplayKit,Xinchi/AsyncDisplayKit,Adlai-Holler/AsyncDisplayKit,Adlai-Holler/AsyncDisplayKit,smyrgl/AsyncDisplayKit,harryworld/AsyncDisplayKit,lappp9/AsyncDisplayKit,levi/AsyncDisplayKit,Xinchi/AsyncDisplayKit,aaronschubert0/AsyncDisplayKit,gazreese/AsyncDisplayKit,chrisdanford/AsyncDisplayKit,rmls/AsyncDisplayKit,lappp9/AsyncDisplayKit,tomizimobile/AsyncDisplayKit,hannahmbanana/AsyncDisplayKit,chrisdanford/AsyncDisplayKit,JetZou/AsyncDisplayKit,tomizimobile/AsyncDisplayKit,marmelroy/AsyncDisplayKit,facebook/AsyncDisplayKit,rmls/AsyncDisplayKit,garrettmoon/AsyncDisplayKit,JetZou/AsyncDisplayKit,nrcmedia/AsyncDisplayKit,harryworld/AsyncDisplayKit,yufenglv/AsyncDisplayKit,samhsiung/AsyncDisplayKit,harryworld/AsyncDisplayKit,flovouin/AsyncDisplayKit,jellenbogen/AsyncDisplayKit,george-gw/AsyncDisplayKit,nrcmedia/AsyncDisplayKit,levi/AsyncDisplayKit,facebook/AsyncDisplayKit,smyrgl/AsyncDisplayKit,smyrgl/AsyncDisplayKit,yufenglv/AsyncDisplayKit,bimawa/AsyncDisplayKit,Adlai-Holler/AsyncDisplayKit,facebook/AsyncDisplayKit,marmelroy/AsyncDisplayKit,garrettmoon/AsyncDisplayKit,Adlai-Holler/AsyncDisplayKit,Xinchi/AsyncDisplayKit,bimawa/AsyncDisplayKit,garrettmoon/AsyncDisplayKit,eanagel/AsyncDisplayKit,romyilano/AsyncDisplayKit,maicki/AsyncDisplayKit,bimawa/AsyncDisplayKit,garrettmoon/AsyncDisplayKit,aaronschubert0/AsyncDisplayKit,nrcmedia/AsyncDisplayKit,maicki/AsyncDisplayKit,rahul-malik/AsyncDisplayKit,levi/AsyncDisplayKit,romyilano/AsyncDisplayKit,jellenbogen/AsyncDisplayKit,rcancro/AsyncDisplayKit,chrisdanford/AsyncDisplayKit,maicki/AsyncDisplayKit,maicki/AsyncDisplayKit,JetZou/AsyncDisplayKit,hannahmbanana/AsyncDisplayKit,aaronschubert0/AsyncDisplayKit,Adlai-Holler/AsyncDisplayKit,rcancro/AsyncDisplayKit,samhsiung/AsyncDisplayKit,rahul-malik/AsyncDisplayKit,george-gw/AsyncDisplayKit,yufenglv/AsyncDisplayKit,romyilano/AsyncDisplayKit,levi/AsyncDisplayKit,rahul-malik/AsyncDisplayKit,tomizimobile/AsyncDisplayKit,hannahmbanana/AsyncDisplayKit,facebook/AsyncDisplayKit,tomizimobile/AsyncDisplayKit,flovouin/AsyncDisplayKit,nrcmedia/AsyncDisplayKit,rcancro/AsyncDisplayKit,smyrgl/AsyncDisplayKit,flovouin/AsyncDisplayKit,marmelroy/AsyncDisplayKit,marmelroy/AsyncDisplayKit,aaronschubert0/AsyncDisplayKit,flovouin/AsyncDisplayKit,jellenbogen/AsyncDisplayKit,yufenglv/AsyncDisplayKit,levi/AsyncDisplayKit,JetZou/AsyncDisplayKit,Xinchi/AsyncDisplayKit,jellenbogen/AsyncDisplayKit,gazreese/AsyncDisplayKit,lappp9/AsyncDisplayKit,eanagel/AsyncDisplayKit,eanagel/AsyncDisplayKit,gazreese/AsyncDisplayKit,eanagel/AsyncDisplayKit,harryworld/AsyncDisplayKit,george-gw/AsyncDisplayKit,rcancro/AsyncDisplayKit,Xinchi/AsyncDisplayKit,rmls/AsyncDisplayKit,flovouin/AsyncDisplayKit