* `WSGIGateway` uses chunked encoding when no `Content-Length` header and both the server and the client use `HTTP/1.1`.
* Removed checking the existence of `Content-Length` header in `Request.write()`.
* Added `iterable` argument to `RequestDone.__init__()` for returning the iterable as WSGI response, PEP:0333.
Closes #717.