I do not believe the problem is with windows.location and your JavaScript. The problems is rather with how gramfeed.com interprets tags. Try this in your code:
var url = "https://www.google.com/search?q=kühl"
window.location = url;
See that special characters stay unconverted.
Now try typing http://gramfeed.com/instagram/tags/kühl
directly in browser address bar - the URL gets converted.