LDAP: case insensitive replace for more robustness

Because the \\5c is brought there by ownCloud it should be enough to do the replacement case sensitive. However, if you do something funny with the database, things can do look different. So, although this problem is not brought by ownCloud and usually does not happen, let us still take care of it.

Quick review @karlitschek @DeepDiver1975 or others please