You have no idea how happy we were when we saw success in the response. It's been a long time coming. Thanks for your insight and help Sanford and Kenny!
Great that it's working. This is actually a bug: the default request type should be based on the encoding setting. But anyway, you can override it like this (and customizing the Content-Type can be crucial in some situations).
Could you DM me with a detailed description of the bugged behavior, Sanford?
That error is on the other side (it's a .Net error, Marketo uses Java).