Rework and enhance fitsheader tests
Rework and enhance fitsheader tests

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