Bug in getYearWeek function

Proposed fix:

function getYearWeek( $date = NULL )
{
if ( NULL === $date )
{
$week = $this->getWeek();
if($week == 1){
$d = new Date($this->getEndOfWeek());
$year = $d->getYear();
}else if($week == 52){
$d = new Date($this->getStartOfWeek());
$year = $d->getYear();
}else{
$year = $this->getYear();
}
return sprintf('%04d%02d', $year, $week);
}

$date = new Date( $date );
return $date->getYearWeek( NULL );
}