It's more tortured than one would imagine
1/ All HTTP GET requests are stripped of headers. It's not in the Flex stack so it's probably the underlying Flash player runtime
2/ All HTTP GET requests that have content type other than "application/x-www-form-urlencoded" are turned into POST requests
3/ All HTTP POST requests that have no actual posted data are turned into GET requests. See 1/ and 2/
4/ All HTTP PUT and HTTP DELETE requests are turned into POST requests. This appears to be a browser limitation that the Flash player is stuck with.
What this boils down to in practical terms is that if you want to pass headers in all requests, you should always use POST and you should find another way to communicate the semantics of the operation you "really wanted". The Rails community have settled on passing ?_method=PUT/DELETE as a work around for the browser problems underlying 4/
Since Flash adds the wonderful header stripping pain on GET, I'm also using ?_method=GET as a workaround for that. However, since this trips up on 3/,
I am passing a dummy object as the encoded POST data. Which means my service needs to ignore dummy posted data on a ?_method=GET request.
Crucial at this point to know about 2/. That wasted a bunch of my time.
I've built all of this handling into a new RESTService class with MXML markup support so it's possible to pretend this doesn't exist on the client side.
Hope this helps someone.