Fixed IE7 prompting the user to download OpenSearch description xml after login (for a private site)

Flow:
1. Browser (IE7) is redirected to the login page.
2. Browser reads the page, sees OpenSearch descriptions, tries to
   download them. Each request gets recorded by SN as the page the user
   should be redirected to after logging in (returnto).
3. User logs in, then gets redirected to the returnto action, which is
   an OpenSearch description.

The OpenSearch descriptions aren't sensitive so making them public in a
private site should be okay.

(I recall fixing this in 0.8.x... :-( )