login.action does not handle ?os_destination parameters in a graceful fashion if the user is already logged in.

If you are already logged in, and you ask confluence for a url like: http://localhost:8080/login.action?os_destination=%2Fdisplay%2Ftest%2FHome
Confluence will inform you:
You do not have permission to access display/test/home. To access this page, please log in as a user with sufficient permissions.

Even if you do in fact have the appropriate permissions.

Steps to reproduce:
1) Copy a url to a page.
2) Log out
3) paste url into browser. It should redirect you to login.action?os_destination=pathtopage
4) copy that url.
5) login.
6) paste url back into browser

Expected output: contents of page
Real output: error regarding permissions.

This was duplicated in 273 and 281.