Describe the bug

Results don't always match the spec when using this gem with also the rails gem loaded (more precisely: the activesupport gem, which is a dependency of rails).

This is because Rails overrides a number of to_json methods and modifies them in a way that is incompatible, for example.

To reproduce
Steps to reproduce the behavior:

1. Install and require this gem and rails
2. "&".to_json_c14n should give "&" but instead gives "\u0026".

Expected behavior
This gem's behavior shouldn't depend on the presence of other gems, at least not ones as common as Rails.