[5.7] Modify ELoquent Api Resource merge() methods to accept JsonResource object

[amaelftah]

this PR , allows both merge & mergeWhen to accept JsonResource object , to remove duplicate of referencing same attributes in different resources .

simplified example of use case , we want to retrieve a user with his profile , but the case is the UserResource mustn't change cause the class has been used in different places in a messy project and could cause api compatibility issues if changed :

class UserResource extends JsonResource
{
     public function toArray($request)
     {
        return ['name' => $this->name , 'age' => $this->age];
     }

}

class UserWithAvatarResource extends JsonResource
{
     public function toArray($request)
     {
       //old way
       return ['name' => $this->name , 'age' => $this->age , 'avatar' => $this->avatar];

        //another way
       return array_merge(
             (new UserResource($this))->jsonSerialize() ,
             ['avatar' => $this->avatar] 
        );

       //when PR accepted
       return [
            $this->merge(new UserResource($this)),
            'avatar' => $this->avatar, 
        ];
     }

}

[bonzai]

You can just do something like this if you want to merge two resources:

class UserWithAvatarResource extends UserResource
{
     public function toArray($request)
     {
        return array_merge(parent::toArray($request), [
            'avatar' => $this->avatar,
        ]);
     }
}

I'm not sure if adding support for JsonSerializable is a good idea (at least in version 5.7). If this would be accepted, I would also add support for Arrayable contract.

• https://github.com/laravel/framework/blob/5.7/src/Illuminate/Support/Collection.php#L1837-L1850

[amaelftah]

@bonzai nice idea 👍 , but in some cases we will need to merge multiple other resources .

i think this isn't a breaking change for 5.7 as it doesn't alter any old behavior