Improved on-the-fly account creation. If some attributes are missing (eg. not present in the LDAP) or are invalid, the registration form is displayed so that the user is able to fill or fix these attributes.

git-svn-id: http://redmine.rubyforge.org/svn/trunk@1678 e93f8b46-1217-0410-a6f0-8f06a7374b81