Better support for HTTPBodyStream

Since NSURL session by default uses HTTPBodyStreams, this is needed to support anything that uses NSURLSession, including AFNetworking 2.0. Whee!
I think the two loops (`for`-`in` over `sortedKeys` and `for` loop with `i`) could be consolidated into a single loop that added the appropriate strings to a mutable array, then post-loop joined the components with `@","`, avoiding the check for the last item... but that'd still leave open the issue of quoting/escaping and nested serialization.