In 'fitsheader -f', fixed bug caused by empty keywords.
In 'fitsheader -f', fixed bug caused by empty keywords.

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