AbstractResultSet::current() change breaks consistency

[Jakovitz]

AbstractResultSet::current() can buffer non-valid data.

Similar example as in #51:

use Zend\Db\Adapter\Adapter;
use Zend\Db\ResultSet\ResultSet;
use Zend\Db\TableGateway\TableGateway;

require 'vendor/autoload.php';

$adapter = new Adapter(array(
    'driver'   => 'pdo_mysql',
    'database' => 'db-album',
    'username' => 'root',
    'password' => ''
));

//using TableGateway instance
$sampleTable = new TableGateway('album', $adapter, null,new ResultSet());
//selecting...
$rowset = $sampleTable->select(array('artist' => 'Non-existent'));
$rowset->buffer();
var_dump($rowset->current()); // null
var_dump($rowset->current()); // false