Invalid signature for service "ProductAdvertisingAPI" due to downcasing of service name

[dallagi]

Hello,
Thanks a lot for developing this library.

When trying to authenticate a request (by signing headers) to the ProductAdvertisingAPI service, AWS always responds with error 401: InvalidSignature.

After some digging, I noticed that this is caused by this line in the AWSAuth.AuthorizationHeader.sign/9 function:

service = String.downcase(service)

If I comment that line, AWS responds back with a successful response (200 OK).

I'm not sure if downcasing the service name is required for other use cases, but if it's not needed and this is indeed a bug, please let me know and I will submit a PR to fix it :)

EDIT: Submitted a PR in case it may be useful