related: ruby-grape/grape#1933

Steps to reproduce:

• clone repo && cd
• run $ bundle install && rake db:create
• start server: $ rails s
• fire request:

curl -X POST \
  'http://localhost:3000/base/sessions?registration[waiting_list_enabled]=true' \
  -H 'Cache-Control: no-cache'

expected response:

{
    "error": "Total capacity must be present"
}

actual response:

{
    "error": "registration[total_capacity] must be present"
}