We use ldap that has no anonymous access and so you cannot search it without authentication. So we could not login using provided code.
After removing search and building user_dn by hand everything works fine, and we can log in using our LDAP :)

This diff replaces: http://reviews.review-board.org/r/696/ . I could not use Update Diff because of some strange form error. 

Basic, on our local server - logging in started to work :)