`web_response.Response.prepared` is `False` after `write_eof()`

[arthurdarcet]

web_response.Response.prepared is False after write_eof() (the payload writer goes back to None).

This is a bit mis-leading - any reason not to change Response.prepared to self._eof_sent or self._payload_writer is not None?

[asvetlov]

This is the oversight, sure.

Please feel free to make a pull request.