Error after payment: Computed checksum does not match header checksum.

Problem/Motivation

The callback from QuickPay after a payment fails in checksum validation producing the following errors on the dblog:

Path: /da/commerce-quickpay/callback. Symfony\Component\HttpKernel\Exception\AccessDeniedHttpException: in Drupal\Core\Routing\AccessAwareRouter->checkAccess() (line 120 of <mysite>/core/lib/Drupal/Core/Routing/AccessAwareRouter.php).

and

Computed checksum does not match header checksum.

Steps to reproduce

1. Install and configure this module
2. Make a payment
3. Inspect the dblog and the QuickPay administration to see the failed callback

Proposed resolution

The docs from QuickPay suggest using the following method for getting the request content:

$request_body = file_get_contents("php://input");

By using this in stead of re-encoding the $content I managed to get the checksum validation to pass.

Remaining tasks

1. Identify the exact cause of the error
2. Make a proper patch