Fixes #6768 : boolean false values at priority list should be valid

Fixes #6768
was meant this:
```
        $this->list->insert('null', null, null);
        $this->list->insert('false', false, null);
        $this->list->insert('string', 'test', 1);
        $this->list->insert('true', true, -1);

        $orders1 = array();
        $orders2 = array();

        foreach ($this->list as $key => $value) {
            $orders1[$this->list->key()] = $this->list->current();
            $orders2[$key] = $value;
        }
        $this->assertEquals($orders1, $orders2);
        $this->assertEquals(
            array(
                'null'   => null,
                'false'  => false,
                'string' => 'test',
                'true'   => true,
            ),
            $orders2
        );
```