Return instance when inserting into a polymorph-related model

This Update makes the return value of MorphOneOrMany::create() Operations consistent with HasOneOrMany.

```php
// Before PR:
HasOneOrMany::create($attributes); // returns instance of model
MorphOneOrMany::create($attributes); // returns bool (save())

// After PR:
HasOneOrMany::create($attributes); // returns instance of model
MorphOneOrMany::create($attributes); // returns instance of model
```