Revert "Revert "Merge pull request #1160 from Shopify/pxpay_credential_based_url""

This reverts commit dbba453cb459a9b06471824a41a8579398f64d76.
Always extract query into form_fields
Quick cleanup
Merge pull request #1169 from Shopify/pxpay_update

Handle URLs without query params for PxPay