fix wrong String.split_at/2 output when second arg is not an integer
Merge pull request #2875 from jw2013/string

fix wrong String.split_at/2 output when second arg is not an integer
fix wrong String.at/2 output when second arg is not an integer
Merge pull request #2877 from jw2013/string

fix wrong String.at/2 output when second arg is not an integer