Fix spiegel.de video

spiegel.de/video didn't work anymore:
```
elias@moria ~ % youtube-dl -g http://www.spiegel.de/video/johann-westhauser-videobotschaft-des-hoehlenforschers-video-1502367.html
ERROR: Failed to download XML: HTTP Error 404: Not Found; please report this issue on https://yt-dl.org/bug . Be sure to call youtube-dl with the --verbose flag and include its complete output. Make sure you are using the latest version; type  youtube-dl -U  to update.
```

This PR fixes it.

Not sure about the test cases, as/if they might need to be adapted as well.