fix sip reading problem with wcs key.
fix sip reading problem with wcs key.

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