Keep referrers for ecommerce tracking in mixed HTTP and HTTPS environment

Question

I use Google Analytics ecommerce:addTransaction to track the revenue after an acquisition.

This works without problems, when my site uses HTTP only.

But since I redirect the client to a SSL page during the buying process I get the payment service provider domains as referrers instead of the real referrers.

This is the Analytics code on the checkout success page:

ga('create', 'UA-*******-1', 'example.com');
ga('require', 'linkid', 'linkid.js');
ga('send', 'pageview');
ga('require', 'ecommerce', 'ecommerce.js');
ga('ecommerce:addTransaction', {
                        'id': '<?php echo $transaction['id']; ?>',
                        'affiliation': '<?php echo $transaction['affiliation']; ?>',
                        'revenue': '<?php echo $transaction['revenue']; ?>',
                        'shipping': '<?php echo $transaction['shipping']; ?>',
                        'tax': '<?php echo $transaction['revenue']; ?>',
                        'currency': 'EUR'
                    });
// adding items in a loop with ecommerce:addItem
ga('ecommerce:send');

The client normally lands on a normal http page. Once redirected to https the client stays on https sites. For example:

1. http://www.example.com

2. https://www.example.com/order

3. https://www.example.com/checkout

4. https://domain.of.paymentprovider.example.net

5. https://www.example.com/success