[5.7] Improve the return value of the caseKey method

[seth-shi]

• this PR is for improving the return value of the caseKey method
• The castkey method should have returned the value of the primary key type, rather than judging by the is_number or is_string method

Example

class User extends Eloquent
{
    protected $table = 'users';
    protected $fillable = ['id', 'email'];
    public $timestamps = false;

    public function articles()
    {
        return $this->belongsToMany(Article::class, 'article_user', 'user_id', 'article_id');
    }
}
class Article extends Eloquent
{
    protected $table = 'articles';
    protected $keyType = 'string';
    public $incrementing = false;
    public $timestamps = false;
    protected $fillable = ['id', 'title'];
}
## users
---------------------------------------------------   
id 
1  
## articles
id      
---------------------------------------------------                                                            
123                                                              
7b7306ae-5a02-46fa-a84c-9538f45c7dd4 
(string)PHP_INT_MAX+1                             

• In the past, I have set the primary key type of the Article model to string, but if I insert an integer overflow number through`$user->articles()->sync(), I can successfully insert the data, but if the type of the return value is a long numeric string, Will not get the right results.

[seth-shi]

@tillkruss Hello, can the style of the test code be beautified through you?

[sisve]

Would these changes be needed in other (read: non-pivot) places too? I'm thinking mostly about consistency; this looks like it will introduce new types for primary keys for pivots, so we should probably do that for normal models too?

[seth-shi]

@sisve The correct type relationship has been handled in the model, as long as keyType is set, the error here is mainly because the primary key value in the parameter is simply used in the code to determine whether it is a number or a string. Rather than judging by the type of primary key of the model

[taylorotwell]

What if the relationship is using a custom pivot model class with its own key type?