exactly_one_of seems not work for nested parameters

Hello,

I'm currently creating an API with the latest version(0.14.0) but exactly_one_of doesn't work as expected for nested parameters.

Here is the sample.

params do
  optional :additionalFacets, type: Hash do
    requires: label, type: String
    requires: facet, type: Hash do
      requires :field, type: String
      requires :type, type: String

      optional :limit, type: Integer
      optional :start, type: Integer

      exactly_one_of :limit, :start
    end
  end
end

In this case I meant the validation of exactly_one_of works only when additionalFacets are given.
But it works when it is not given as well.

Can you help check this?

Start by reproducing the nested-parameter example from the issue with Grape 0.14.0 and trace the exactly_one_of validation entry point. Done means exactly_one_of is enforced when additionalFacets is supplied, but is not triggered when that optional parent parameter is absent; add a regression test for both cases.