[8.x] Modify mergeCasts to return $this

[finagin]

This change helpful to use the method in pipeline like this:

    public function initializeUsesUuid()
    {
        $this
            ->setKeyName('uuid')
            ->setKeyType('string')
            ->setIncrementing(false)
            ->mergeCasts([$this->getKeyName() => $this->getKeyType()])
            ->mergeGuarded([$this->getKeyName()]);
    }

[GrahamCampbell]

Please send to master, since this is a BC break (making a public method return something where it was void before in a non-final class).

[driesvints]

@GrahamCampbell this isn't a breaking change