Basically, this commit brings a brand new Response API to the party:
   - Response::send() can be used to send a fixed-size content and
     will use the classic Content-Length header
   - When content size is not known in advance, a Stream interface can
     be used to stream the response. A ResponseStream is obtaining by a
     call to the Response::stream member function. After that, headers
     will be immutable and data can be streamed a-la-ostream with
     operator<<