fix bug in z_at_value
fix bug in z_at_value

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