extra check for type coercion for issue #19

in case pin sneaks through without being coerced into a string?

I just noticed this during a meeting, but I haven't had a chance to test this yet, please feel free to ignore this for the time being and I'll test it more thoroughly.  -- Meant to help with this thread: https://github.com/rwaldron/spark-io/issues/19#issuecomment-45295181