Add check for np.ma.nomask in NDData mask setter.
Add check for np.ma.nomask in NDData mask setter.

Closes #2784

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