LdapSource cannot create raw record string from non-ascii data

When the data returned from the LDAP directory query includes non-ascii characters, the call to json_encode() in the retrieve method of LdapSourceBackend fails to return a string and instead returns a false that is ignored. The result is that the array returned by retrieve() does not have a useful value for or representation of the source record stored in the 'raw' key.

Note that Active Directory in particular will almost always return an LDAP record that contains non-ascii characters.