Once a response has been written to, the status and headers are supposed
to have been sent over the network. So any future change to them is moot.
Likewise, setBody does nothing: only write can continue appending to the
existing body.
As in the case of WSGI the response does not actually get sent just yet:
- lock the body
- store finalize() result
Also, make the order in which body and stdout are returned consistent with
HTTPResponse: body is returned first (as part of HTTPResponse.write's call
to self.outputBody, which calls self.__str__, which emits body), and then
stdout content.
Also, add support for cases whre the existing body is a file or an
IUnboundStreamIterator.