Fix user groups resolution when user entry references groups with the wrong case

Steps to reproduce:

• set up a case insensitive ldap directory for users and groups, with user entries referencing group dns
• each of the groups is found in the ldap, even if the group id does not have the same case => the group id is kept on the user principal for rights resolutions
• the group is given permissions on the repo tree

=> the user, even if he is a member of the group, will not have the appropriate permissions because the groups referenced in the principal and the group in ACL do not have the same case (and security checks are case sensitive)