No URL encoding in sitemaps

[anton-tsyganenko]

Nanoc SitemapXML helper doesn't encode URLs that contain non-ascii-characters, but that is required.
https://support.google.com/webmasters/answer/183668
For example, if I create create a file called проверка.html, the sitemap looks like this:

 <url>
    <loc>https://example.com/проверка/</loc>
    <lastmod>2017-01-06</lastmod>
  </url>

But it has to look like this:

 <url>
    <loc>https://example.com/%D0%BF%D1%80%D0%BE%D0%B2%D0%B5%D1%80%D0%BA%D0%B0/</loc>
    <lastmod>2017-01-06</lastmod>
  </url>