Fix Phalcon\Cache\Backend\Libmemcached::get not returning frontend values

More info here: http://forum.phalconphp.com/discussion/3855/phalcon-20-libmemcached-problem#C12829
Allow model::toArray with no parameters
Merge pull request #2978 from rzajac/2.0.0

Fix Phalcon\Cache\Backend\Libmemcached::get not returning frontend value...