Fix rspec failure on X-Inertia requests

[iatanas0v]

As initial request and subsequent requests are handled with different renderers sometimes we may end up in situation where controller action may fail just in one of those cases:

https://github.com/inertiajs/inertia-rails/blob/master/lib/inertia_rails/renderer.rb#L19L28

This is exactly what happened on one of my work projects. Subsequent requests (with X-Inertia header were failing) but my rspec specs were all 🟢 .

Naturally, I decided to add another spec that issues the request with X-Inertia header. It turned out that the rspec helper is not expecting that 😅 . This PR aims to resolve that and allow tests like the one below.

it 'works on subsequential requests' do
  get homepage_path, headers: {'X-Inertia' => true}

  expect(inertia_props.keys).to include('records', 'pagination', 'filters')
end

[BrandonShar]

Thanks @iatanas0v and sorry for taking so long to review this, I"m not sure how I missed it!