Huh.. You are indeed correct - sorry, I didn't thoroughly retest. My bad.
The script is trying to do an OAuth 2 login to a CRM system. Google says, and says only, that the error I'm getting means it's missing the correct Content-Type header in the POST. I changed the URL http://httpbin.org/post and the right headers are in there.
Weird. Time for more digging, I guess. TY!