4.2-fix-associate-on-morphTo

Fix associate to work correctly when `protected $morphClass` is overriden on the Model (after `getMorphClass()` method was added).

```
namespace My\Space;
class Category extends \Eloquent {
  protected $morphClass = 'Category';
}

// currently (using get_class):
$category = My\Space\Category::first();
$tag->taggable()->associate($category);
$tag->taggable_type; // 'My\\Space\\Category'

// should be:
$tag->taggable_type; // 'Category'
```