Enforce string usage with ldap

The singleton could be a fake record and so it will return None for all fields.
ldap requires string as parameters otherwise it raise a TypeError.

issue4159
review12561002
(grafted from 7f2a7c129fde1d97fc86589316011734a38f1a4c)

--HG--
branch : 2.8
extra : source : 7f2a7c129fde1d97fc86589316011734a38f1a4c