Flex :: Http - Prevent From Caching An External Resource

I'm writing a flex application that polls an xml file on the server to check for updated data every few seconds, and I'm having trouble preventing it from caching the data and failing to respond to it being updated.

I've attempted to set headers using the IIS control panel to use the following, without any luck:

CacheControl: no-cache
Pragma: no-cache

I've also attempted adding a random HTTP GET parameter to the end of the request URL, but that seems like it's stripped off by the HttpService class before the request is made. Here's the code to implement it:

http.url = "test.xml?time=" + new Date().getMilliseconds();

And here's the debug log that makes me think it failed:

(mx.messaging.messages::HTTPRequestMessage)#0
body = (Object)#1
clientId = (null)

[Code].....