User is still logged in after logging out

[danielmai]

Queries still succeed as the logged in user even after clicking the log out button.

Steps to reproduce:

1. Run Dgraph cluster with ACLs enabled.
2. Create a user and group with read permissions for a particular predicate.
3. Log in as that user.
4. Run query for that predicate. The query should be authorized and succeed.
5. Log out.
6. Run the same query. The query succeeds, but it should fail since no one is logged in.

Additional notes:

A "log out" works if I log out and then click the Refresh button of the web browser.