AbstractFormBuilder#fields_for should respect the builder of parent

[namusyaka]

Now:

form_for(@user, '/register', builder: "AbstractFormBuilder") do |f|
  f #=> AbstractFormBuilder
  f.fields_for(:role_types, @user.role_types) do |field|
    field #=> StandardFormBuilder
    field.submit_block "Submit"
  end
end

I'd expected this:

form_for(@user, '/register', builder: "AbstractFormBuilder") do |f|
  f #=> AbstractFormBuilder
  f.fields_for(:role_types, @user.role_types) do |field|
    field #=> AbstractFormBuilder
    field.submit_block "Submit"
  end
end

[nesquena]

Nice, thanks!