Bugfix: when checking for an integer, check for both Fixnum AND Bignum

When checking for an integer in the json scheme, we compare its Ruby class to the expected type.
According to [1], we cannot predict whether Bignum or Fixnum will be created as an output for a fixed integer.
As a matter of fact, checking for an integer should check for both classes. This solves https://github.com/tent/tent-admin/issues/1

[1] http://blog.headius.com/2012/10/so-you-want-to-optimize-ruby.html