Add escaping for bad platform string.

[flohdot]

Mitigates XSS on bad download URLs such as https://YOUR.NUTS.URL/download/version/2.0.0/%3Cimg%20src=x%20onerror=alert(1)%3E and https://YOUR.NUTS.URL/download/channel/alpha/%3Cimg%20src=x%20onerror=alert(1)%3E

I don't see anywhere I could just insert a test for this in the existing suite.

[wallymathieu]

Thanks!

[loprima-l]

Hi, I merged the project to a new repo to start maintain it, I would be glad if you can put your pull request here : https://github.com/loprima-l/nuts-2