.toString() throws error when new Table was instanciated with "head" prop as undefined.

The interface for ```TableOptions``` suggests that is it is valid to pass the constructor options head property as undefined, as if it were not passed at all - standard JS/TS practice.

Unfortunately this causes the ```.toString()``` method to throw an error since it does not appear to be checking whether the head options property isn't undefined.

I think this should just require a quick gaurd to ensure this is handled correctly?