ISO-8859-1 encoding, bad suggestion
The proxy upgrade seems to suggest that you have decided to use ISO-8859-1 instead of utf-8 for all services from trafiklab.
This sounds like a really bad idea since its not very future proof. If you break the API, better do it so you don't have to do it again in a year time.
I don't understand why you would not convert your ISO-8859-1 text to utf8 in the proxy, this is a very cheap process (there are various highly optimized versions in assembler out there) and it ensures that you can include characters as basic as the euro €.
Its just good practice to follow what the (internet) industry is doing, which is converting to using utf-8 in all the places of interoperation. Its just future-proof.