Flexible response

This is to address Olivier Ligot's need for filter and CORS
What it does is allowing to set the status code and the header  as long as no content is really sent.
And it should not penalize performance to send content.
It might slow things down just a little bit when putting header, but this is usually not the critical part of web server application.

While doing this patch, I thought about going even more user friendly by allowing the app to send content without putting the Content-Length

for http/1.1  this could be done using chunked encoding if no Content-Length is specified.
or we could just buffer the content, and send everything at the end of the request processing.
But .. this would be for another patch.