Test that you can reopen nested exposures

Related to https://github.com/intridea/grape-entity/pull/62

The test was actually not right. The exposures inside the :user exposure in the root entity `Person` are getting overwritten in the children.


````ruby
class Person < Grape::Entity
##This gets overwritten by the exposure in Student. So there is no `:in_first` exposure.
  expose :user do
    expose(:in_first) { |_| 'value' }
  end
end

class Student < Person
  expose :user do
    expose(:user_id) { |_| 'value' }
    expose(:user_display_id, as: :display_id) { |_| 'value' }
  end
end
```

I corrected the test to reproduce the issue.