[BugFix] Little bug fix inside product model

You could not set a master variant if the non-master variant was already added to the product. This patch fixed it.
Sorry, I'm sleepy so I can be wrong =) but what about?

```php
if (!$masterVariant->isMaster()) {
    $masterVariant->setMaster(true);
}

if (!$this->variants->contains($masterVariant)) {
    $masterVariant->setProduct($this);
    $this->variants->add($masterVariant);
}
```
ps. phpspec would be your friend here =)