Fix mongo handler

Hello,

First of all, tests pass :

    php run-tests.php Session
    > OK, but incomplete or skipped tests!
    > Tests: 568, Assertions: 781, Skipped: 1

This fix is about the last version of php-mongo driver, where safe option is deprecated.
More informations here : http://www.php.net/manual/fr/mongo.writeconcerns.php#mongo.writeconcerns.options

On my dev environnement (archlinux), a lot of "deprecated" messages are present without this fix.