bugfix: using symbols as keys in hash as well as strings as keysin hash

Found this bug when I was trying out table_print in irb with a hash. I put symbols as keys, as below:

>data = [{name: 'Bob', age: '1'}]
>tp data

This failed with a method not found error. Changing the keys to strings, got me the column names but not the values. Anyway I think the fix is pretty straight forward...