Fix "method_complexity" issue in lib/mini_api/default_responder.rb

[leoncruz]

Method transform_keys has a Cognitive Complexity of 6 (exceeds 5 allowed). Consider refactoring.

https://codeclimate.com/github/leoncruz/mini_api/lib/mini_api/default_responder.rb#issue_64926afac9fcb60001000023