[DoctrineBridge] Loosened CollectionToArrayTransformer::transform() to accept arrays

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Previously, writing an association getter like this was impossible:

```php
public function addTag(Tag $tag) { ... }
public function removeTag(Tag $tag) { ... }

public function getTags()
{
    return $this->tags->toArray();
}
```

Using `toArray()` is a useful way to restrict modifications of the collection to the specified methods. But previously, CollectionToArrayTransformer failed in this case, because it did not accept arrays as input.