[dm-validations] :format validator now works with fixnums
[dm-validations] :format validator now works with fixnums

* Always calling #to_s on the value to validate leads to
  4 spec errors, therefore why I added the check if value
  is_a?(Fixnum)

[#859 state:resolved]
mbj/aequitas,emmanuel/aequitas,datamapper/dm-validations