Fix world_keep when supplied a SlicedLowLevelWCS object
Fix world_keep when supplied a SlicedLowLevelWCS object

If we had a split coupled SLLWCS it broke

mhvk/astropy,larrybradley/astropy,dhomeier/astropy,lpsinger/astropy,saimn/astropy,StuartLittlefair/astropy,lpsinger/astropy,mhvk/astropy,aleksandr-bakanov/astropy,pllim/astropy,StuartLittlefair/astropy,astropy/astropy,astropy/astropy,lpsinger/astropy,pllim/astropy,dhomeier/astropy,aleksandr-bakanov/astropy,pllim/astropy,astropy/astropy,dhomeier/astropy,StuartLittlefair/astropy,larrybradley/astropy,astropy/astropy,MSeifert04/astropy,larrybradley/astropy,aleksandr-bakanov/astropy,mhvk/astropy,MSeifert04/astropy,pllim/astropy,stargaser/astropy,StuartLittlefair/astropy,saimn/astropy,larrybradley/astropy,astropy/astropy,aleksandr-bakanov/astropy,stargaser/astropy,mhvk/astropy,lpsinger/astropy,StuartLittlefair/astropy,stargaser/astropy,pllim/astropy,dhomeier/astropy,saimn/astropy,stargaser/astropy,saimn/astropy,mhvk/astropy,lpsinger/astropy,MSeifert04/astropy,dhomeier/astropy,MSeifert04/astropy,larrybradley/astropy,saimn/astropy