Fixes a type error for cases where an AN field is made up of all numbers

Hi,

I _think_ I've found a very small bug in the way that alpha-numeric fields are parsed.

In the case where the an AN field is entirely numeric it can be passed in as type Fixnum.  Under ruby 1.9 this won't be cast to string when =~ is called, we end up with an exception being thrown.

I've added a couple of explicit to_s calls to fix this this.

Cheers,
Sean.