Fix fixtures path for model generator when using namespaced models

[ghost]

When fixures are enabled the model generator create the fixture files with an incorrect path for namespaced models

application.rb configuration

config.generators do |g|
  g.test_framework :rspec, fixture: true
end

Running the generator for a namespaced model

bin/rails g model admin/posts

Expected

spec/fixtures/admin/posts.yml

Actual

spec/fixtures/admin_posts.yml

[benoittgt]

Hello @nxlith

This PR looks good to me. Fails on CI seem unrelated (maybe: rails/rails#35153, rails/rails#35154).
I will try to fix the CI if I find time.

[benoittgt]

PR to fix the build. #2078

[benoittgt]

Do you think you can pull rebase master?

[ghost]

Hi @benoittgt, rebase done

[benoittgt]

Thanks a lot @nxlith! 🎉

LGTM. Waiting for a second approval from @JonRowe or @samphippen :)