Make parameter for SphinxQL->update() optional!

[KarelWintersky]

Make parameter for 'update' method optional, because method replace have no required param:

public function replace()
    {
        $this->reset();
        $this->type = 'replace';

        return $this;
    }

usable as: $connection->replace()->into($index)->...

But:

public function update($index)
    {
        $this->reset();
        $this->type = 'update';
        $this->into($index);

        return $this;
    }

REQUIRES $index and can't be used same way