fix path helpers returning nil with certain arguments

@gi-lunaweb unfortunately I introduced a bug in my last PR (the bug only occurs when using the host_locales option). The path helpers were incorrectly returning nil when given anything other than a hash as an argument. This PR fixes the bug and adds a test for it.

I also moved locale_param_key back to lib/route_translator.rb, I had moved this to lib/route_translator/host.rb but I think it makes more sense in its original position.