Fixes and additions from https://github.com/edqu-ab.
- Replaces Initialize7 with Initialize8 handler.
- Adds AddSingleOrderLine2 and PurchaseInvoiceCorporate handlers.
- Adds pypayex specific alternative names of proxy variables.
Thanks to alexbrasetvik for these patches.
- Added PxCredit5 and PxCheck2 handlers.
- Properly deal with dicts and lists when normalizing.
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