option to disable DEFLATE encoding

I ran into a problem serving compressed requests to Internet Explorer because of the zlib wrapper included with DEFLATE'd responses which, while correct according to the RFC, is not supported by IE. https://connect.microsoft.com/IE/feedback/details/1007412/interop-wininet-does-not-support-content-encoding-deflate-properly . In lieu of doing some user-agent detection or always removing the wrapper, it would be nice to have an option to disable DEFLATE usage in favor of gzip. I'm completely willing to submit a PR but wanted to open an issue first for discussion.