If the first request points to a path-based subwiki then navigating to the main wiki afterwards displays a permission error

To reproduce:

• have a mutiki setup (virtual=1 with at least one subwiki created) with path-based and you've logged in on the main wiki with the Admin user by typing "Admin" as the user
• Stop your wiki
• Start it and enter a URL pointing to a subwiki
• Then go to the main wiki and verify you get a permission error and the wrong skin

The issue is that when hitting the first URL the user saved in the session will be <subwiki>.XWiki.<youruser> and when you navigate to the main wiki this user will not exist and thus not have permissions.