[11.x] json assertions on streamed content

[gdebrauwer]

Currently it is not possible to use all json assertions on a streamed json response. This PR fixes that.

Route::get('/users', function () {
    return response()->streamJson([
        'data' => User::cursor(),
    ]);
});

$this->getJson('/users')
    ->assertJsonCount(10, 'data')
    ->assertJsonPath('data.*.id', $users->pluck('id')->all());