Message pack improperly serializes maps with numerical indexes

[joryacooper]

When emitting a map of data that uses non-sequential numerical indexes, e.g.:

$data = [
    "3" => "foo",
    "7" => "bar"
];
$emitter = new SocketIO\Emitter(); 
$emitter->emit('event', $data);

The data is serialized, emitted, and received as an array, e.g.:

{
    "0": "foo",
    "1": "bar"
}

The only way around this right now is to add a non-numerical key into the map, e.g.:

$data = [
    "3" => "foo",
    "7" => "bar",
    "hack" => true
];
$emitter->emit('event', $data);

The reason this is happening is because in this file (https://github.com/rase-/socket.io-php-emitter/blob/master/src/msgpack_pack.php#L79) starting at line 79, the code is converting the data to an array if all keys in the data are ints, regardless of if the keys are sequential or not.