Fix select clause on belongsToMany relation

Tiny fix on BelongsToMany relation addressing #2679.

Let's you do this:

```
// User model, belongsToMany('Category')

$user->categories()->select('name', updated_at')->get();
// or
public function categoriesAnotherRelation()
{
  return $this->belongsToMany('Category')->select('name', 'updated_at');
}

// resulting query:
// 'SELECT name, updated_at, pivot_fields... ' 
// instead of:
// 'SELECT name, updated_at, categories.*, pivot_fields... '
```