updated default arguments of Unit to not use None, which is now invalid
updated default arguments of Unit to not use None, which is now invalid

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