I need to shorten a protocol relative url such as //www.example.com/longurl into another protocol relative url //short/url so an iframe can be embedded on a page independent of the used protocol. Are there any free URL shortening services that allow this? I've tried isgd, tinyurl, googl, bitly, owly among others, but they all either don't accept the url or prepend it with http:// so it turns into http:////www.example.com/longurl.
It's undocument, but it turns out that goo.gl supports this, you just have to modify the URL it gives you, although it seems the URLs I tried were longer than what they accept.