Fix named routing regression from 3.2.13

When named route that is nested is used in 3.2.13

Example `routes.rb`:

```
resources :nested do
  resources :builder, :controller => 'nested/builder'
end
```

In 3.2.12 and 3.2.12 this named route would work:

```
nested_builder_path(:last_step, :nested_id => "foo")
```

Generating a url that looks like `/nested/foo/builder/last_step`. This PR fixes the regression when building urls via the optimized helper. Any explicit keys set in the options are removed from the list of implicitly mapped keys.

Not sure if this is exactly how the original version worked, but this fixes this use case regression.