Check that item is string when creating a new table column
Check that item is string when creating a new table column

Previous version broke setting rows with t[1] = (values) because
it tried to create a new column 1.  Instead make sure the item is
a string and is therefore referring to a column name.

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