Add test for HDU writeto using various file modes
Add test for HDU writeto using various file modes

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