Fix issues if authentication is done by web server

This patch fixes `user` always being None if the authentication is
offloaded to the webserver, as it is suggested in the documentation.

For normal access, this is not a problem, but it becomes a problem if a
client wants to get the current-user-principal, for which the user name
is required.
Merge pull request #99 from horazont/fix-preauth

Fix issues if authentication is done by web server