This works as expected in Chrome. However, when using this request format in Firefox, the headers are sent but the POST (form) data is not. Obviously this makes the call useless.
I’m not aware of any such issues. Maybe you could file a bug with a repro case - and post the case number here for us to take a look? (Note that all of Unity R&D is doing a HackWeek this year, and we will not get to look at it before next week).