Update MpesaHelper.php

[steveyout]

No description provided.

[Iankumu]

I am not able to reproduce the issue raised in this pr . Also this pr introduces breaking changes.
The best option would be to call $response->json() from your side rather than introduce it within the package and explicitly format the response to be returned as an associative array.

Example

use Iankumu\Mpesa\Facades\Mpesa;
$response=Mpesa::stkpush($phonenumber, $amount, $account_number,$callbackurl = null);

 /** @var \Illuminate\Http\Client\Response $response */
$result = $response->json(); // provides the json decoded data as an associative array
return $result;