Computing path payment price is probably using wrong rounding

[vogel]

Path payment computation on horizon uses rounding down:

it uses truncating division at the moment:
https://github.com/stellar/go/blob/master/services/horizon/internal/simplepath/order_book.go#L113

The pricing mechanism for a single path element can be found here:
https://github.com/stellar/go/blob/master/services/horizon/internal/simplepath/order_book.go#L24

And in stellar-core it uses rounding up: https://github.com/stellar/stellar-core/blob/master/src/transactions/OfferExchange.cpp#L91

Here are some tests in stellar-core:
https://github.com/stellar/stellar-core/blob/master/src/transactions/ExchangeTests.cpp