Не совсем понял что ты хочешь сделать. Но чтоб получить требуемый запрос, сделай так:
Started POST "/subscribtions/validate" for my_ip at Tue Oct 27 18:13:49 +0400 2015
DEPRECATION WARNING: Setting filter_parameter_logging in ActionController is deprecated and has no longer effect, please set 'config.filter_parameters' in config/application.rb instead. (called from /app/controllers/application_controller.rb:5)
Processing by SubscribtionsController#validate as JSON
Parameters: {"subscribtion"=>{"email"=>"nedam@mail.com"}}
Completed 200 OK in 5ms (Views: 0.1ms | ActiveRecord: 0.0ms)