Move Pxpay to use credential_based_url
Use mocks in Pxpay helper tests
Add option for passing a customer email to Pxpay helper
raise ArgumentErrors vs Runtime in Pxpay initializer
Fix up Pxpay remote tests
Adapt Klarna to take line items with types like 'shipping'
Merge pull request #1160 from Shopify/pxpay_credential_based_url

Pxpay credential based url
Merge pull request #1161 from Shopify/klarna-fixes

Adapt Klarna to take line items with types like 'shipping'
Revert "Merge pull request #1160 from Shopify/pxpay_credential_based_url"

This reverts commit 49148eb93c381a4ccc62a1703677814f13582084, reversing
changes made to ed26b96a65097bddf80c78a8ff2e2b760250bd5f.