added additional validation of nddata initialization
added additional validation of nddata initialization

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