code: protected void firstWrite()
          throws IOException
      {
          if (_response.isCommitted())
              return;
          
          // Nobble the OutputStream for HEAD requests
          if (HttpRequest.__HEAD.equals(_request.getMethod()))
              _outputStream.nullOutput();    
              
          int length=_response.getIntField(HttpFields.__ContentLength);
          if (length>=0)
              _outputStream.setContentLength(length);
      }
docs: Setup the reponse output stream.
  Use the current state of the request and response, to set tranfer
  parameters such as chunking and content length.