`redirect` helper is mutating arguments

This change I'm less sure about. For the sake of semantics I [redirected `http_prefix`][config] instead of `'/'`, but the extension's path normalization actually changed that setting.

Would you consider this to be a bug in the extension, or should I just be duplicating the setting before I pass it in?

  [config]: https://github.com/AndrewKvalheim/opt.outof.link/blob/116161a9bc64d456d3d402aeb808565709106dc8/config.rb#L22