make coordinates copy if passed into initializer of same coordinate
make coordinates copy if passed into initializer of same coordinate

closes #1727

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