[9.x] Do not transform JsonSerializable instances to array in HTTP client methods

[JaZo]

This is a follow-up of #41055.

Problem

When a class implements both Arrayable and JsonSerializable, the former currently takes precedence. However, a class can have different forms for Arrayable and JsonSerializable. For example, an empty hashmap should be an empty array in array form, but an empty object in JSON form i.e. {"attributes":{}}. This is especially important if the endpoint always expects an object, even if it is empty. So I'd expect that if I post an object as JSON, and setup the correct jsonSerialize method, it'll be serialized as such, not transformed to an array.

Solution

This PR will skip transforming the request data, only in JSON body format, if it implements JsonSerializable and thus passes it 1:1 to Guzzle for JSON serialization.

Change

Given the following class:

class Foo implements JsonSerializable, Arrayable
{
    public function jsonSerialize(): mixed
    {
        return [
            'attributes' => (object) [],
        ];
    }

    public function toArray(): array
    {
        return [
            'attributes' => [],
        ];
    }
}

Before

Http::post('http://example.com', new Foo); // posts {"attributes": []}

After

Http::post('http://example.com', new Foo); // posts {"attributes": {}}