Double URL-encoding bug after login

[mathiasbynens]

Reproduction steps

1. Log out
2. Open e.g. https://devwebfeed.appspot.com/admin/stats?author=Mathias%2BBynens
3. Go through the login flow

Expected result:

You’re now at https://devwebfeed.appspot.com/admin/stats?author=Mathias%2BBynens.

Actual result:

You’re now at https://devwebfeed.appspot.com/admin/stats?author%3DMathias%2BBynens. Note how the = got incorrectly escaped as %3D.