Fix broken configuration when module_type is Nil and namespace is being used.

[rapito]

Hi!

I have a simple project that doesn't use any import/export modules and is still using sprockets. On latest version, if I have this configuration:

JsRoutes.setup do |c|
  c.module_type = nil
  c.namespace = 'Routes'
end

It breaks like this when I load any endpoint:

After reviewing, I noticed that 'DTS' was being used by default causing this clash. I solved it by monkey patching it on my project, but I thought I would suggest this modification to support the intended behavior of prioritizing DTS only if the configuration_type has not been set.