GetCompressionType Cross Platform fix.

Updated `GetCompressionType` to use the `Accept-Encoding` header directly for determining the compression type, this makes this method work as expected regardless of target platform.

Currently the method throws a `NotImplemented` exception when calling the current `IRequest.RequestPreferences` method is called because it relies on the, not implemented, `System.Web.HttpContextWrapper.GetService ` method in XSP/fastcgi-server-x Mono implementations. 

This issue has been raised [http://stackoverflow.com/a/21294048/1295344](here), and again [here](http://stackoverflow.com/q/22648844/1295344). And prevents use of compression features on some Mono hosted environments.