fixes #6327 - carry along differential_cls in ._apply
fixes #6327 - carry along differential_cls in ._apply

dhomeier/astropy,AustereCuriosity/astropy,larrybradley/astropy,funbaker/astropy,saimn/astropy,larrybradley/astropy,saimn/astropy,larrybradley/astropy,stargaser/astropy,dhomeier/astropy,stargaser/astropy,aleksandr-bakanov/astropy,lpsinger/astropy,AustereCuriosity/astropy,larrybradley/astropy,StuartLittlefair/astropy,mhvk/astropy,mhvk/astropy,pllim/astropy,mhvk/astropy,aleksandr-bakanov/astropy,funbaker/astropy,lpsinger/astropy,dhomeier/astropy,StuartLittlefair/astropy,bsipocz/astropy,MSeifert04/astropy,AustereCuriosity/astropy,funbaker/astropy,saimn/astropy,astropy/astropy,MSeifert04/astropy,DougBurke/astropy,pllim/astropy,funbaker/astropy,MSeifert04/astropy,pllim/astropy,mhvk/astropy,bsipocz/astropy,StuartLittlefair/astropy,DougBurke/astropy,aleksandr-bakanov/astropy,saimn/astropy,lpsinger/astropy,pllim/astropy,lpsinger/astropy,stargaser/astropy,dhomeier/astropy,astropy/astropy,astropy/astropy,saimn/astropy,dhomeier/astropy,bsipocz/astropy,StuartLittlefair/astropy,bsipocz/astropy,AustereCuriosity/astropy,stargaser/astropy,DougBurke/astropy,StuartLittlefair/astropy,AustereCuriosity/astropy,astropy/astropy,MSeifert04/astropy,DougBurke/astropy,mhvk/astropy,aleksandr-bakanov/astropy,pllim/astropy,astropy/astropy,lpsinger/astropy,larrybradley/astropy