It may be a bit late to answer this, but I did a little research using Httpful and found the answer. Httpful uses the default set of handlers for each mime type. If someone is registered before sending a request, he will use the one that you registered. Conveniently, there is a class Httpful\Handlers\JsonHandler. The constructor takes an array of arguments. The only one he uses is this $decode_as_array. Therefore, you can force it to return an array as follows:
$json_handler = new Httpful\Handlers\JsonHandler(array('decode_as_array' => true));
Httpful\Httpful::register('application/json', $json_handler);
$response = Request::get('some-url')->send();
UPDATE
, funky-, , JSON. , , . , , , JSON :
$response = Request::get('some/awesome/url')
->expects('application/json')
->send();