Fix functional user tests

cauth fails if the back argument is given as a query string and POST parameter,
because value in clean_back is now a list and no longer a string.

This fixes the functional tests, not clean_back; to me it is ok if clean_back
fails because a user should never send the back arguments as query string and
POST parameter (because it is unknown which argument is the correct one).

Change-Id: I4f5f60c06c2c41bdcf53d7898b034d4ae38a9e76