Fix cascade reading at Grape::API's level

[ericproulx]

This PR fixes the call to cascade when read from the api.

The following test was failing

describe '.cascade' do
  subject { api.cascade }
  let(:api) do 
    Class.new(Grape::API) do
      cascade true
    end
  end

  it { is_expected.to eq(true)}
end

All make proper changes after #2633 is merged since the latter is moving the function in routing module