Only compute the CSRF_COOKIE when it is actually used. This is a
significant speedup for clients not using cookies.
Changed result of the “test_token_node_no_csrf_cookie” test: It gets
a valid CSRF token now which seems like the correct behavior.
Changed auth_tests.test_views.LoginTest.test_login_csrf_rotate to
use get_token() to trigger CSRF cookie inclusion instead of changing
request.META["CSRF_COOKIE_USED"] directly.
