Setting correct charset on Pingdom API POST requests

I'm in the process of updating my code base using a newer REST client for request to the Pingdom API. Previous version worked fine, the only difference seems to be that the new POST request contains a "charset" value on the content-type. Old version without worked fine, new version with charset fails, returning "400, Required parameter missing: type".

All the "type" parameters I'm aware of are present in the POST request, and I've tried all the charset values I'm aware of (UTF-8, UTF-16, UTF-32, Latin1, ASCII).

Does anyone know the correct charset value for the Pingdom APIs, or if the presence of a "charset" requires extra parameters?