Bug 15889: LDAP authentication: Only update mapped attributes

Bug 15889: LDAP authentication: Only update mapped attributes

Test plan:

- Update your configuration file to use LDAP authentication and enable update
  (<update>1</update>) option,
- login with an existing user with extended attrbitutes that are not in
LDAP mapping,
- check that all attributes are still here.

Signed-off-by: Chris <chrisc@catalyst.net.nz>
Signed-off-by: Philippe Blouin <philippe.blouin@inlibro.com>
Signed-off-by: Jesse Weaver <jweaver@bywatersolutions.com>
(cherry picked from commit c74678a1d239aaf91906039ccb3db940df4a0b41)
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>