Simple versioning is not dead

Fixes #507
you stopped the sanity check if the document is versionable at all. you should do something like:

```php
if (! $metadata->versionable) {
    throw new InvalidArgumentException(sprintf("The document at '%s' is not full versionable", $path));
}
$node = $this->session->getNode($path);
$mixin = $metadata->versionable === 'simple' ? 'mix:simpleVersionable' : 'mix:versionable';
if (!$node->isNodeType($mixin)) {
    $node->addMixin($mixin);
}
```

the check with the if is to not additionaly add simpleVersionable if the node already is versionable.

ideally we would have some more tests, for example about error conditions like a non-versionable node. also a fully versionable node should be tested to work as well.