v1.4.2: Make search_dn / search_password optional

The documentation already stated that the parameters search_dn and search_password were optional but in fact they were not because an exception was thrown when the initial LDAP binding (using search_dn / search_password) did not work. This release uses user's input credentials for LDAP binding if binding with search_dn / search_password did not work (or these have not been set) making a global LDAP user which can access all credentials not necessary anymore.