[9.x] Fix inconsistent content type when using ResponseSequence

[ksassnowski]

This PR fixes an inconsistency when using the ResponseSequence::push method.

Summary

The \Illuminate\Http\Client\Factory::response() method sets the response’s Content-Type header to application/json and JSON encodes the response body if an array was passed as the $body parameter.

Http::fake(fn () => Http::response(['foo' => 'bar']));
$response = Http::get('http://example.com');
$response->header('Content-Type');
// => "application/json"

The push method of the ResponseSequence internally calls Factory::response to create a new Response object. However, the ResponseSequence::push method also JSON encodes the request body if an array was passed before calling Factory::response. This means that the Content-Type header will not get set to application/json in this case because $body will be a string instead of an array.

Http::fakeSequence()->push(['foo' => 'bar']);
$response = Http::get('http://example.com');
$response->header('Content-Type');
// => ""

The fix is to simply not JSON serialize the $body variable inside ResponseSequence::push as Factory::response already does that.