`relLangURL` for mail and ftp produce invalid URL

I use relLangURL on render-links.html hook. But when the links contain mailto and ftp, it return invalid URL. How ever it worked if contain http or https.

My render-links.html:

<a href="{{ relLangURL .Destination }}">{{ .Text }}</a>

Example markdown input:

[Send me email](mailto:email@example.com)
[Download File](ftp://example.com/file.txt)
[Visit Site](https://example.com)

The result:


For now I tweaked render-links.html like this:

{{ $destination := .Destination }}
{{ if not (hasPrefix $destination "mailto") }}
  {{ $destination = relLangURL .Destination }}
{{ end }}

<a href="{{ $destination }}">{{ .Text }}</a>

It worked for mailto link, but not for ftp.

I think It would better if this handled by relLangURL function.

This is a bug in the relURL and relLangURL functions. We will fix this.