Unset the IMS header when the LoggedOut cookie is present

This allows caching for pages the client doesn't have in cache.

Change-Id: I7314dc6e27c5c8a2c8245ee4f335fd8eda2e684e
Fix vcl_error function name (case)

Change-Id: Id6d20722650228147a3c150f2471f0a78dbd5e1a