paypro: fix signature error. fix merchant_data. see #1409.
paypro: fix invalid signature error.
paypro: minor fix to createPaymentTx callback.
paypro: fix error passing to abide by new flow. see #1410.
test: update tests for paypro fix.
paypro: compensate for improperly implemented payment protocol servers.
Merge pull request #1410 from chjj/fix_paypro_sent

paypro: fix signature error. fix merchant_data. see #1409.