state parameter for authorize_with_service_account should be optional

[vlladdrakk]

I've recently upgraded the this gem in a project from 0.37 to 0.41 and just got an error in production when using authorize_with_service_account without having a state parameter. I see that the state parameter was added in 0.38 (#104) and there was no documentation added to the method describing what it is for. According to the API documentation this is an optional parameter, but it was not made optional when added to this SDK.
I need to improve my testing around our use of this gem and when I upgrade the version, but I also think that this parameter needs to be made optional to accurately reflect the API.