We're functioning as a WSGI server here, so this bit from PEP-3333 seems
to apply:
> The start_response callable must not actually transmit the response
> headers. Instead, it must store them for the server or gateway to
> transmit only after the first iteration of the application return
> value that yields a non-empty bytestrin ... . In other words, response
> headers must not be sent until there is actual body data available, or
> until the application's returned iterable is exhausted.
Plus, it mirrors what swob.Request.call_application does.
Change-Id: I1e8501f8ce91ea912780db64fee1c56bef809a98
f7a820ed3a