Fix route helper to work with string params

Before Rails 4.2.0, `model_path("id" => 1)` would work fine. As of Rails 4.2.0, it generated a couple of errors:

1. `ArgumentError: comparison of Array with Array failed` (due to attempting to sort a hash with mixed string/symbol keys in the case where an `UrlGenerationError` should have occurred)
2. `ActionController::UrlGenerationError`, because of missing required key :id (i.e., it ignored the `"id"` string parameter)

This fix is on 4-2-stable, so I think this would need to be ported to the master branch as well.