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!
Isaac and I spent an hour yesterday trying to open it directly, no dice. It'll only get you the data in the delegate methods. I don't think you can really set an XCTestExpectation within a non XCTestCase subclass, so this was pretty much the only way that wouldn't block the read operation.